jenkins1986 JCPDS International Centre for Diffraction Data Sample Preparation Methods in X Ray Powder Diffraction

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JCPDS — International Centre for Diffraction Data
Sample Preparation Methods in X-Ray Powder Diffraction
JCPDS Data Collection and Analysis Subcommittee —
Dr. R. Jenkins, Chairman
Sample Preparation Section Task Group
Members:
Dr. T. G. Fawcett, The Dow Chemical Co.
Prof. D. K. Smith, The Pennsylvania State University
Dr. J. W. Visser, Technisch Physische Dienst, Delft,
The Netherlands
Ms. M. C. Morris, National Bureau of Standards
Dr. L. K. Frevel, The Dow Chemical Co. (Ret.) and
Johns Hopkins University
Introduction
The aim of any diffraction experiment is to obtain
reproducible data of high accuracy and precision so that the
data can be correctly interpreted and analyzed. Various
methods of sample preparation have been devised so that
reproducibility, precision and accuracy can be obtained.
The success of a diffraction experiment will often depend on
the correct choice of preparation method for the sample be-
ing analyzed and for the instrument being used in the
analysis.
A diffraction pattern contains three types of useful in-
formation: the positions of the diffraction maxima, the peak
intensities, and the intensity distribution as a function of dif-
fraction angle. This information can be used to identify and
quantify the contents of the sample, as well as to calculate
the material's crystallite size and distribution, crystallinity,
and stress and strain. The ideal preparation for a given ex-
periment depends largely on information desired. A sample
preparation which is used only for the identification of its
phases may be quite different from a preparation used to
measure strain, which in turn may be different from a prep-
aration used in quantitative analysis. The following sections
will review various sample preparation techniques which are
used to optimize different types of diffraction information.
The General Considerations, Section 1, will discuss sample
preparation problems which are common to every diffrac-
tion experiment. The Instrument Specific Methods, Section
2, will outline sample preparation methods which are com-
mon to various instrumental configurations i.e. Gandolfi,
Debye-Scherrer, and Guinier cameras and various diffrac-
tometers.
1. General Considerations
There are many aspects of a sample which affect the quality
of the diffraction data obtainable from that sample. Positions
of diffraction maxima, their intensity and shape are all sen-
sitive to sample factors. These factors include crystallite and
particle size, size and physical shape of the sample, position
of the sample on the instrument, crystallite orientation, and
crystallite and particle absorption. Each of these factors will
be discussed separately including sample preparation tech-
niques designed to minimize deleterious effects. As a "rule-
of-thumb* the best sample preparation methods are those
which allow the analyst to obtain the desired information
with the least amount of sample treatment. Samples used to
obtain accurate d values may be treated very differently
from the samples used for intensity measurements.
Care must be taken in any sample treatment not to
alter the physical state of the sample under study. Polymor-
phic inversion or decomposition may be induced during
grinding. Hydrated compounds may gain or lose water by
exposure to the atmosphere or a liquid dispersant used to
distribute the powder on a sample support. Pharmaceuticals
and organics are especially sensitive to sample treatments
and mixtures may be even more susceptible than pure
samples, since a physically harder material may act as a
grinding agent on a softer phase. Sample preparation itself
may require a controlled atmosphere enclosure such as a
glove box or a glove bag. Some aspects of sample prepara-
tion may have to be skipped in order to assure no change in
the sample. The analyst may have to obtain a preliminary
set of diffraction data on a sample prior to any treatments
as a comparison to assure no subsequent sample changes
occurred.
Table 1 taken from a recent review by Jenkins1, lists
some of the possible changes which may occur during the
preparation of the specimen or during the actual analysis.
By far the greatest potential source of problems is in grind-
ing the sample where the introduction of strain and amor-
phism is common and where under certain circumstances,
polymorphic and compositional changes may also occur.
Decomposition and polymerization may occur during the ir-
radiation of the specimen particularly where weak ligands
are present in the compounds making up the material under
investigation. Finally, the atmosphere immediately sur-
rounding the specimen during analysis may create problems
of loss or gain of water of hydration, carbon dioxide, etc. Of
these three sources of difficulty, the analyst really has control
only of the third, where the use of controlled atmospheres
can be employed.
Table 1.
Possible Causes for Compositional Variations Between the As-Received
Sample and the Prepared Specimen
1. Induced by Grinding
a) Amorphism
b) Strain
c) Decomposition (e.g., loss of CO2)
d) Polymorphic change
e) Solid state reaction
f) Contamination by mortar
2. Induced by Irradiation
a) Polymerization
b) Decomposition
c) Amorphism
3. Special Problems
a) Hydration, carbonation, etc., due to atmospheric
conditions
b) Loss of water in vacuum
c) Decomposition at high temperature
The fact that polymorphic changes may be induced by
grinding has long been recognized, but the problem may be
more common than is generally recognized by many prac-
ticing diffractionists. Table 2 gives a short list of some of the
cases reported in the literature and includes examples of
polymorphic changes, decomposition and formation of solid
Powder Dijfraction, Vol. 1, No, 2, June 1986 51
solutions. Several of the indicated polymorphic changes are
reversible, for example the transformations between calcite
and aragonite, and litharge and massicot.
Mills used for reducing the particle size of samples for
powder diffractometry are generally of the ball mill (e.g. the
Wig-L-Bug) or disc mill (e.g. Shatterbox) types. The stress
system generated in these mills is a combination of hydro-
static and shear stresses) of which the shear stress is generally
the more important. Further, although local heating may
certainly accelerate any solid state interaction, the major fac-
tor in any phase change is purely a mechanical one. It has
been estimated that the pressure component in a Wig-L-
Bug is between 10-20 kbars (one bar = 106 dynes/cm =1
atmosphere) and one would intuitively assume that the
pressure generated in a disc mill is considerably in excess of
this value. This should be considered in the light of the fact
that many of the listed phase transformations occur at much
less than 10 kbars, for example the calcite-aragonite trans-
formation will commence at 3 kbars and the lead dioxide
transformation at 9 kbars.
Table 2.
Reported Polymorphic
from Grinding*
Initial Phase
Calcite (CaCO3,
rhombohedral)
Vaterite (CaCO3,
hexagonal)
Litharge (PbO,
tetragonal)
Lead dioxide
(PbO2, tetragonal)
Kaolinite (A12(OH)4
Si2O5, tridinic)
Boehmite
(y-AlO(OH),
orthorhombic)
Wurtzite (ZnS,
hexagonal)
KC1 plus KBr
Antimony plus
Bismuth
Massicot (PbO, plus
Sulphur
ZnCO3
CdCO 3
Montmorillonite
Changes and Solid State
Final Phase
Aragonite (CaCO3,
orthorhombic)
Calcite (CaCO3,
rhombohedral)
Massicot (PbO,
orthorhombic)
Lead dioxide
(PbO2, orthorhombic)
Mullite (3A12O3
2SiO2, orthorhombic)
y-Alumina (y-Al2O3,
hexagonal) plus
Corundum (o-Al2O3
rhombohedral)
Sphalerite (ZnS,
cubic)
solid solution
solid solution
PbS plus SO2
Loss of CO2 to form
basic carbonates
A12O3, MgO, etc.
Reactions Resulti
Reference
Burns & Bredig2
Schrader and Hoff-
man3 Dandurand4
Gregg5
Northwood and
Lewis6 ,
Lewis et al7
Senna &Kuno8
Dachille & Roy9
Takahashi10
Panis11
Gregg5
Vegard & Hauge12
Dandurand
Lin14
Burton15
Bloch16
*Note: It should be pointed out that some of the indicated phase changes
are reversible.
References 2-4 show the degree of conversion of calcite and
aragonite as a function of grinding time. In the case of
calcite/aragonite, with a grinding of 2-20 minutes, the max-
imum transformation would probably not exceed a few per-
cent but in the case of a less stable system such as
massicot/litharge, the possible transformation could be
several tens of percent.
Samples which are reactive require special handling. If
the analyst has a diffraction system available with a con-
trolled atmosphere chamber, the only problem is to transfer
successfully the sample to that chamber. Otherwise, the
sample will require special packaging to preserve integrity
while still allowing the X-ray beam to see the sample sur-
face. A large number of tricks to protect samples have been
devised and will be discussed in a subsequent section.
Samples which are to be heated during the study require
supports which are inert. Precious metals such as platinum
are commonly employed, but alternative materials may be
more suitable in some cases. Low temperature samples may
actually be formed in situ or supported on some conductive
substrate. These special considerations will also be discussed
later.
A. Crystallite Size
Crystallite size and particle size in an X-ray sample may not
always be synonymous, so one must be careful in the use of
these terms. Crystallite size refers to the size of the individual
domains which diffract the X-rays coherently. Particle size
refers to the grain size in a powdered state. Individual grains
may be made up of many crystallites, thus, particle size of a
sample may be many times the crystallite size. Most size ef-
fects in diffraction experiments are due to the crystallites.
Particle size is often misquoted in diffraction literature when
crystallite size is meant. The particle size may actually be
the whole sample when a metal or ceramic block is used. In
general, it is the crystallite size which is important when one
considers the orientation effects in the sample, the peak
shapes and the noise levels in the background. Particle size
becomes important when analyzing absorption effects, and
then only when the sample is composed of loosely packed
particles.
Crystallite size and particle size are essentially
synonymous when large single crystal grains are crushed to
prepare a powder for analysis. The size effects of crushed
grains has been reviewed by Klug and Alexander17 and
more recently by Parrish and Huang18. The intensity of an
X-ray beam is measured in photons (counts) per second. All
other things being constant, this counting process follows the
laws of statistics, i.e. the variation in the number of counts
detected is measured by the standard deviation which is the
square root of the number of counts measured. The dif-
fracted beam intensity measured is a function of this varia-
tion and any variation in the number of crystallites in a
moving sample which are in a position to diffract the beam
in any time interval. If the crystallite size of a sample
is large, there are too few crystallites to provide equal
probability of diffraction for all sample orientations, and the
variation of the intensity values measured increases
significantly. Figure 1, reproduced from reference 18, shows
the variation of intensity of a selected diffraction peak as a
function of the crystallite size. During the data collection the
sample is rotated which causes different crystallites to con-
tribute to the diffraction of the beam. The variation of the
observed intensity approaches the variation due only to
counting statistics only when the crystallite size is in the
5 jum or smaller range. This result indicates that this size
range is necessary for accurate intensity measurements in
any sample. Achieving this size range is easier said than
52 Powder Diffraction, Vol. / , No. 2, June 1986
34.0
10-20 pm (311) >30/Jm (111)
Figure 1. Recordings of intensify variations of rotating silicon powder samples of various particle sizes.
accomplished.
The exact relationship between an optimum crystallite
size and the appearance of the diffraction peaks in the ex-
perimental pattern depends on the material which makes up
the sample, but empirical measurements imply that the best
and most reproducible diffraction traces are obtained when
the size is less than 10 \jm and preferably around 1 \xm, see
Table 3. A 0.5 cm2 surface of sample particles with a grain
size of 10 / m will yield approximately 106 particles in the ir-
radiated area in a diffractometer geometry. Smaller crystal-
lites will increase this number accordingly. For samples
where random grain orientation and accurate intensity
measurements are the goal, small grain sizes are essential.
However, there is a lower limit to the crystallite size as well
where smaller crystallites cause the peaks to broaden. This
limit is around 0.2 \Jtxn. Where intensity measurements do
not have to be perfect, a size range of around 30-50 /Ltm will
prove to be satisfactory. This coarser size will usually yield
sharp peak shapes whose positions are most easily located.
The JCPDS Associateship at the National Bureau of
Standards has developed a procedure which they follow,
when the sample permits in the production of reference
standards. Morris et al. *9>20
 u s e ^ Q different samples when
obtaining reference data. One sample is used for obtaining
d-values, and another smaller particle size (10 \Jtn\ or less)
sample is used for intensity measurements. In this case the
optimum size to obtain accurate d-spacing is different from
the optimum size needed for the most accurate intensities.
Edmonds et al.2* point out the fact that in quantitative
analyses, standards should be of the same particle size as the
material to be analyzed so that factors such as crystallinity
and microabsorption can be the same for the sample and the
reference material.
The grain size needed will determine the extent of
grinding. The analyst must be careful that the grinding pro-
cedure does not decompose the sample. If the material is
soft, such as cellulose, then grinding can reduce the sample's
crystallinity22, Figure 2. Care must be taken when mixtures
are being ground because the harder materials in the mix-
ture may grind softer materials and the grinding process
may destroy the soft material's crystallinity without reducing
the size of the hard materials. In most cases when working
with mixtures and unknowns the analyst may have a trade-
off between optimized grain size and varying sample
crystallinity.
If the sample is to be ground, care must be taken so
that the grinding apparatus does not contaminate the sam-
ple. Mortars and pestles made of boron or tungsten carbides
usually reduce the probability of contamination. Soft
materials such as organic compounds, malleable metals and
fibrous or platy compounds may be difficult to prepare with
a mortar and pestle.
Powder Diffraction, Vol. 1, No. 2, June 1986 53
Table 3.
Intensity Measurements on Different Size Fractions of <.325-Mesh
Quartz Powder [145]*.
Specimen 15 to 50jim 5 to 50^m 5 to 15^m <5^m
No. Fraction Fraction Fraction Fraction
1 7.612 8.688 10.841 11.055
2 8.373 9.040 11.336 11.040
3 8.255 10.232 11.046 11.386
4 9.333 9.333 11.597 11.212
5 4.823 8.530 11.541 11.460
6 11.123 8.617 11.336 11.260
7 11.051 11.598 11.686 11.241
8 5.773 7.818 11.288 11.428
9 8.527 8.021 11.126 11.406
10 10.255 10.190 10.878 11.444
Mean area 8.513 9.227 11.268 11.293
Mean deviation 1.545 929 236 132
Mean percentage
deviation 18.2 10.1 2.1 1.2
* Tabulated values are areas in arbitrary units of the 3.34-A maximum as
counted with a Geiger-counter diffractometer usingCuKa radiation.
Diffractometer angle 20°
1012141618 20222426 1012141618 2022 24 26
7 6 5 4.5 4 3.5 8 7 6 5 4.5 4 3.5
Interplanar distance, Angstrom units
Figure 2. Sample grinding reduces particle size leading to line widlh broadening.
Calcined at 350°.
(001) (002) (100)
Calcined at 375°—
(001)
Calcined at 700°-
(001) (002) A (100)
Calcined at 1200°—
(001) A(002) (100)
Crystallite
Size
50-85A
150-340A
550-1010A
Figure 3. X-ray diffraction patterns of calcined beryllium oxide showing effect of crystallite
size vs. half peak breadth for various diffraction angles.
54 Powder Diffraction, Vol. 1. So. 2, June] 986
When the crystallites are less than 0.2 ftfn broadening
will develop in the peak profiles. The width relationship with
crystallite size was investigated by Rau2 4 among others.
Figure 3 shows data obtained in reference 24, on a cali-
brated diffractometer, and indicates the type of data an
analyst may observe in a diffraction scan. Similar data along
with discussions on the interrelationship between crystallite
size and peak width can be found in references 17 and 55.
The interrelationship between width and crystallite size
can be influenced by variables such as instrumental con-
figuration and sample stress. One can see that when
crystallite sizes are very small, the analyst may have a dif-
ficult time determining both the true peak positions and
their intensities. Extensive broadening, especially in peak
clusters may make it difficult or impossible to get a correct
value for the true baseline signal. The broadening itself
tends to create more clusters which can decrease resolution
and reduce the detectability of phases in a mixture. Depend-
ing on the information desired in a particular diffraction
scan, an optimum crystallite size may be necessary.
Grinding the sample can reduce the effective crystallite
size to the range which causes broadening as evidenced by
the increasing peak widths observed in Figure 2. The
analyst must be careful when grinding not to reduce the
crystallite size to the detection limit of the diffraction experi-
ment. A common way to increase the crystallite size or
reduce any strain is to heat the sample so that the crystallites
grow and anneal. Care must be taken if the sample is heated
in situ because the crystals may orient themselves in the
sample or on the sample holder (see orientation l.C). If the
sample is heated externally in an oven, then the sample may
be lightly reground to reduce orientation before being
analyzed. The analyst should take a diffraction scan before
and after the heating process to insure that the sample has
not undergone a thermally induced phase transition. Recrys-
tallization in a different crystallization medium from the
original sample may result in a more beneficial crystallite
size. Polymers are often mechanically and/or thermally
treated during crystallization to promote crystallite growth.
The number of crystallites in the samples has to be suf-
ficiently large as to generate enough signal to give reproduci-
ble intensities for every peak for each phase in the diffraction
pattern. The reduction of the particle size and crystallite size
allows more crystallites to be in a position to diffract per unit
volume of the sample. If the instrumental apparatus allows
the analyst to irradiate a larger sample, then a stronger
signal will result. However, in this case a larger sample's ab-
sorption effects and profile resolution may become a prob-
lem. Poorer profiles may result from an inability of keeping
a large sample in alignment with the X-ray optics.
B. Orientation
In qualitative and quantitative analyses of unknown
materials, the analyst must eliminate preferred orientation
of the crystallites so that the diffraction pattern can be
matched to standards for identification. Materials with
equant crystal shapes and no cleavages that produce shaped
grains usually yield random samples using any sample
preparation method. As the difference between the max-
imum and minimum dimensions of the particle increases it
becomes more difficult to make a random sample. Fibrous
shapes are generally more difficult to randomize than platy
shapes. Vassamillet and King indicate that the single most
important factor in achieving randomness is to have a fine
particle size25.
One method used to reduce orientation is to dilute and
blend the sample with spherical particles which can prevent
the sample from becoming oriented. The JCPDS Associ-
ateship of the National Bureau of Standards (Morris etalJ9)
uses finely ground silica gel. Glass spheres and starch have
also been used due to their spherical shapes.
An excellent method for achieving randomness in the
crystallite orientation is to spray-dry the sample. In this
method a powder is suspended in a liquid containing a
deflocculent and a binder, see reference 23 and references
therein. The mixture is sprayed into a heated chamber and
spherical aggregates are formed which dry before falling into
a collection surface. The operating conditions of the spray
drier are critical to the effectiveness of the sample prepara-
tion. While this method of sample preparation is time con-
suming (at least initially) and requires additional expense of
the spray drier, high quality diffraction patterns can be ob-
tained by this method. The analyst must be careful that the
sample does not interact with the deflocculent and binder.
Figure 4 shows a sample of MOO3 which was platy
and produced a highly oriented pattern before spray drying
but a random pattern aiter2^. Another method is to mix the
sample with a viscous binder and to mount the mixture into
the sample holder. Several of the authors commonly use
Vaseline or Apiezon grease which is viscous and does not
move during the experiment. Vaseline can also protect
hydrated samples from dehydration and usually does not
react or recrystallize the original sample. Other binders vary
from wetting agents such as acetone, alcohol, water, amyl
acetate, oil and ethers to more viscous materials such as
Duco cement, collodion in amyl acetate or ether, Ambroid
in nitrobenzene, mucilage in Karo, or rubber cement in
toluene. Very dry samples can be dusted through a sieve
directly onto a layer of petroleum jelly, hydrocarbon or
silicone grease. Any binder should be used sparingly
because the presence of binder reduces the concentration of
the sample per unit volume in the X-ray beam. Binders will
also contribute to the general scattering background if they
are non-crystalline or they may give rise to their own diffrac-
tion pattern, such as that of Vaseline. The binder may mask
poorly crystalline materials or phases of low concentration.
Because some binders are difficult to remove, the use of a
binder may prevent sample recovery. Thin binders such as
acetone may allow the particles to settle and shaped particles
to align. Bloss et al.26 describe a method for lightly spraying
fine powders with an atomized binder such as clear acrylic
lacquer. The droplets pull particles into spherical clumps
which randomize their orientation. In atomization methods
(including spray drying) binders such as water, collodion in
a fast evaporating solvent, rubber cement in tolune, and lac-
quer have been used.
Best results are obtained if the spray is directed upward
so that the droplets dry at the peak of the fountain where
they are most spherical.
Most diffraction equipment comes with some type of
optional sample rotation device. Some analysts think the
way to reduce oriented peaks is by rotation of the sample
Powder Diffraction, Vol. 1, No. 2, June 1986 55
5^m
Figure 4. Spherical agglomerate of MoO'3 produced by liquid phase spherical agglomera-
tion.
during the experiment. In fact, rotation of any sample in the
plane containing the Bragg planes which contribute to the
diffracted beam has no effecton sample orientation. Rota-
tion at an angle to the Bragg planes has some effect. The
purpose of sample rotation is to increase the effective volume
of sample seen by the X-ray beam and to bring more crys-
tallites in a position to diffract for each peak. While the sam-
ple rotation may increase particle statistics with a decrease in
the noise level, it will not alter other variables which can af-
fect the quality of the data. A perfect sample rotator would
rotate the sample in 3 directions simultaneously. Commer-
cial sample rotators usually move the sample in either 1 or 2
directions; even the best commercial rotators, therefore, do
not completely randomize the sample orientation. The rota-
tor may randomize one or two directions depending upon its
mechanical motion. While a sample rotator may provide a
means of reducing the effects of particle size and orientation
on the diffraction data, it does not reduce orientation in the
sample and should not eliminate sample preparation
methods necessary to provide a random sample.
In many cases it is desirable to analyze an oriented
sample. In crystalline polymers such as polyethylene or
polypropylene, the analyst may want to determine the
degree of chain alignment relative to the coordinate system
of the sample because this data may relate to the modulus
and tensile properties of the material being analyzed. In soils
analyses the detection of clay minerals in the samples can be
increased by inducing orientation in the sample. For quan-
titative analyses by X-ray diffraction, detection limits can
often be reduced by using oriented samples. In these cases
standard quantity versus intensity curves must be prepared.
It should be remembered that for JCPDS reference stan-
dards the intensities were taken on randomized samples.
For samples with tendencies to orient, standard reproduci-
ble preparation procedures can be employed. The method
of Copeland and Bragg2 ^ has long been used in cement
analyses and a procedure for analyzing respirable quartz has
been established for air quality control. The important
features of these methods are the rigorous adherence to a
specified stepwise procedure.
Clay minerals rarely show strong diffraction effects
from Bragg planes other than the (00!), so it is highly ad-
vantageous to prepare oriented samples. In general, these
samples can be prepared similarly to the method of Gibbs28,
where the sample is slurried with distilled water. The water
is then allowed to evaporate until the slurry is thickened and
then the slurry is smeared into a sample holder. If the slurry
is too thin, then different sized particles may produce an
uneven distribution of phases within the sample. Similarly
the methods of Shaw and Drever can be used2^'30 where the
sample is placed in a suspension and particle sized by set-
tling or filtering through a porous membrane. By removing
particles greater than 2 microns the clays are usually con-
centrated and separated from other materials in the sample.
If the sample is filtered through a small membrane then
often times the membrane can be directly mounted onto the
data collection apparatus. Edmonds and Henslee21 have
shown that if thin samples are deposited on a filter then the
correct choice of filter membrane may become crucial to the
analytical results. In their analyses of respirable quartz they
found that some membrane materials contain pores which
can preferentially orient the samples and change the true
detector response.
The U.S. Geological Survey has done a compilation5^
of random and oriented sample preparation techniques.
Reference 56 provides a series of methods preferred by the
U.S.G.S. to concentrate clay phases and selectively remove
common interferants (silica, quartz, carbonates, oxides).
An interesting method for orienting fibrous phases has
been described by Birks et al.,^ and M. Fatemi et al.,^. In
56 Powder Diffraction, Vol. 1, \'o. 2, June 1986
this technique the fibers are aligned nearly parallel to each
other by an electrostatic process. The fibers are dispersed in
water with a dispersing agent and a sonic generator. An ali-
quot is vacuum filtered onto a filter and the filter is then
ashed. The residue is suspended in a weak solution of parlo-
dion in amyl acetate and sonically dispersed again. A drop
of solution is then spread over a grid of parallel wires which
are alternately bussed, and 240 VAC are applied to the grid.
When the solution has evaporated, a more concentrated
solution of parlodion in amyl acetate is applied to the sam-
ple, the sample is allowed to dry again and then it is peeled
from the grid. The fibers align perpendicular to the wires.
C. Absorption
Sample absorption of X-rays will diminish the diffraction in-
tensities in any given experiment. The amount of absorp-
tion can be limited by the proper choice of sample prepara-
tion technique. Equation 1 describes the absorption of
X-rays by a sample of thickness t.
I = I * - " 1 (1)
t is the sample thickness, pL is the linear absorption coeffi-
cient, Io is the incoming beam intensity and I is the intensity
after the beam transmits through the sample. The linear ab-
sorption coefficient depends on the mass absorption coeffi-
cient, \JQ, the wavelength of radiation used and the density
and elemental composition of the sample according to
Equation 2, where Pn/100 is Pi % of element 1 and P 2 % of
element 2, etc.
(2)
Reference 33 gives an easy to use tabulation of the mass ab-
sorption coefficients of the known elements for several types
of radiation wavelengths. If the composition of the sample is
known, the analyst can calculate the absorbing ability of the
sample to be analyzed. In diffractometer reflection geometry,
absorption is a constant over the whole diffraction pattern
for any given sample. This is not the case for other sample
geometries and recording techniques. Even in the diffrac-
tometer, sample absorption affects the relative intensities of
different phases in a mixture, so it cannot be ignored. One
effect of absorption is to limit the depth of penetration of the
X-ray beam into a sample. An interesting parameter to con-
sider in diffractometer samples is the "half-depth" of penetra-
tion, i.e. the horizon below the surface above which 50% of
the diffracted beam originates. This thickness is approx-
imately the reciprocal of the linear absorption coefficient, \i.
For example, A12O3 (\i = 125 cm - ] ) has a half depth of 80
\*m. UO2 has a half depth of 6 /ion which is less than one
grain thickness in the typical sample. Knowledge of the half
depth of penetration of a sample has considerable influence
on the preparation of the sample. Highly absorbing com-
pounds can be prepared as a thin film on any substrate,
whereas low absorbing materials require thick samples in a
cavity type mount. Consideration of this factor is particular-
ly important in preparing samples for quantitative analysis.
The equations show that the sample absorption in
transmission geometry can be reduced by using thin
samples or changing the sample density in the X-ray beam.
An important consideration is the angle at which the X-rays
meet the sample because this angle changes the path length
of the X-ray beam in the sample which changes the half
depth of penetration. Table 4 shows the interaction of radia-
tion type, incident angle (angle between the incident beam
and the sample surface) and mass absorption for three fairly
common materials. Table 5 shows the interaction of sample
thickness, radiation, and absorption. One can see from
these tables that due to absorption, the sample preparation
methods can vary from instrument to instrument (Section
2) because the radiation wavelength, incident angle and op-
tical pathway, i.e., transmission or reflection from the sam-
ple to the detection device, also varies from instrument to in-
strument. These factors not onlyvary the absorption but
also affect surface sensitivity and are the basis for grazing
angle diffraction experiments. In grazing experiments the
incident angle to the sample is kept below 5°. The lower the
angle the smaller the half depth of the penetration which
results in increased surface sensitivity.
One way to control absorption effects is to reduce the
particle size because large particles more completely absorb
the incoming X-rays. Figure 5 shows an example of the in-
teraction between the particle size and the linear absorption
coefficient and how these factors affect the diffraction inten-
sity1^. This effect is known as microabsorption and is an ef-
fect of particle size not crystallite size. These data were
generated for a diffractometer in reflection geometry. The
magnitude of the deviation will be different for different
samples or geometries and is a very important factor in the
analysis of mixtures with different particle sizes.
Absorption effects should also be considered if a binder
or a substrate is used in the diffraction experiment. In
transmission experiments the thickness and elemental com-
position of the substrate have to be carefully considered so
that the substrate does not absorb or block the incoming
X-ray beam. Due to their low mass absorption coefficients
for most X-ray wavelengths, Be foils are often used as X-ray
tube windows and as X-ray transparent seals on vacuum or
controlled atmosphere devices. However, the toxicity of Be
prevents its common use as a sample substrate. Some of the
authors have used very thin (75 microns) Al foil, and others
commonly use plastic films such as Mylar, 3M Magic
Transparent Tape, or other hydrocarbon films as sample
substrates. As mentioned previously, the analyst has to be
careful that the diffraction pattern of the substrate does not
grossly interfere with the pattern of the sample.
Many problems arise in quantitative analysis due to
the absorption effects in the sample. Each phase in the sam-
ple absorbs X-rays according to its own linear absorption
coefficient, but the beam also passes through all the other
grains in the sample complicating the absorption process.
The effect is to enhance the diffraction of the highly absorb-
ing phases and to mask the diffraction of the weakly absorb-
ing phases. The result causes large departures from linearity
in the response of diffraction intensity to the amount present
in a sample and diminishes the detectability of the weakly
absorbing phases. The effect is so severe that 50% BeO is
very difficult to detect in a BeO-UO2 mixture, whereas
<1 % UO2 is easily detected for the UO2 phase.
There are two common techniques used in quantita-
tive analysis, the internal standard addition method and the
Powder Diffraction, Vol. ], No. 2, June 1986 57
Table 4.
Absorption Values of an Incident Angle of 45° for Polyethylene, Laid and Magnetite.
% Absorption
Sample
Polyethylene
Lead
Fe3O4
Radiation (Kal)
Cu
Fe
Cu
Fe
Cu
Fe
99%
1.1 cm
.59 cm
17.4 ym
10.0 fan
40.5 urn
852.8 fjm
50%
1.7 mm
0.9 mm
2.63 nm
1.50 fan
6.10 urn
128 fan
10%
0.26 mm
0.14 mm
4000 A
2302 A
9243 A
19.5 pm
Table 5.
Depth Penetration at 50% Initial Intensity Absorption for Various Grazing Angles
Sample
Polyethylene
Lead
Fe3O4
Radiation (Kol)
Cu
Fe
Cu
Fe
Cu
Fe
45°
1.1 x 107A
5.8 x 106A
17068 A
9735 A
4.0 x 105 A
8.3 x 106A
10°
2.6 x 106A
1.4 x 106A
4103 A
2340A
9516 A
2.0 x 106A
1.32 x 105A
7.0 x 105A
2050 A
1170 A
4760 A
1.0 x 105A
.5°
13.2 x 104A
7.0 x 104A
205 A
117 A
476 A
9984 A
10 15 20 25 30 35 40
Crystallite dimension (microns)
45 50
Figure 5. Interaction between particle size and linear absorption coefficient, and the rela-
tion of these factors to diffraction intensify.
reference intensity method34. Both techniques compensate
for (but not eliminate) the absorption effects, but they have
no effect on the detectability limits or the counting errors
due to the small amounts of the sample which can be
detected. Anyone who has tried quantifying a low absorbing
phase in a highly absorbing matrix (such as AI2O3 in iron
oxides) has been frustrated by the large errors caused by the
absorption. When using either method, large errors are also
caused by the microabsorption if the linear absorption coeffi-
cients of the phases differ significantly and the grain sizes dif-
fer also. The addition of an amorphous low absorbing
diluent may be useful in this situation. The object of the
diluent is to separate the particles so that they do not in-
terfere with each other. All samples must be made the same
way. In the internal standard method the reference samples
must be treated the same as the samples to be analyzed. In
the reference intensity method, the reference intensity ratios
should be measured by the analyst using samples with the
same physical characteristics as the samples to be analyzed.
Published RIR (reference intensity ratios) values are not
sufficiently universal to be used except as checks on
measured values.
D. Sample Position
Recent systematic investigations36'37 of the common errors
associated with search-matching procedures for identifying
unknowns have shown sample positioning to be the primary
cause for failures in the identification process. Correct sam-
ple positioning depends on both proper instrumental align-
ment (not covered here) and on sample preparation. Incor-
rect sample positioning can be identified and corrected for
by use of internal standards37'3^3^. In a JCPDS round
robin analysis40 where different laboratories analyzed the
same sample for positional accuracy, five of the top six data
sets (out of 22) used internal standards. Errors as large as
0.10° 2-theta were found in data sets not using internal stan-
dards. Sample positioning errors were cited as the largest
contributor to positional inaccuracies40. For samples of beta-
Spodumene (a lithium aluminum silicate), one of the
samples used in the above references, a displacement of 100
microns corresponds to an error of 0.06° 2-theta. External
standards can be used to adjust the data for mechanical er-
rors and instrumental alignment but cannot correct the data
for sample position errors.
Absorption affects the sample position because the
ability of the X-rays to penetrate the sample will vary with
the elemental composition of the sample and the half depth
at which the X-rays are diffracted will also vary. In theory, it
is the half depth of penetration which should be aligned on
the diffractometer axis. As shown in Tables 4 and 5, if a
reflection geometry is chosen in the experiment, most dif-
58 Powder Diffraction, Vol. 1, No. 2, June 1986
fraction from a piece of lead will occur in the first several
thousand A of the sample while most diffraction from a
sample of polyethylene will occur much deeper into the sam-
ple. Therefore, the average position of diffraction in the
sample will vary with composition. We have also shown in
previous sections that the particle size, shape and orientation
can also change the absorption and thus the average position
of diffraction. Many sample preparation methods in Section
2 try to control the sample thickness and particle size and
distribution so that the sample position can be controlled.
As mentioned above, an internal standard can be used
to mathematically correct for sample position errors. Several
high quality standards are available from the National
Bureau of Standards*1 and other sources which give the
analyst the ability to pick the best standard for a particular
sample matrix. Standards are available which calibrate for
positions and intensities. The most common standard is
NBS SRM 640 or 640a silicon powder. These materials have
been used in round robin testing38-42 and have been found
to give highly reproducible results in anumber of analyses.
When choosing a reference material, the analyst must make
sure that the particle size, orientation and absorbance are
known. The analyst should choose a standard which gives
the fewest interfering lines while still bracketing the data at
both the high and low angles. The bracketing of experimen-
tal data with standard data is important because many in-
strumental errors are non-linear in two-theta17. Most of the
current standards are metals or refractories so they are hard
materials; therefore it is recommended that the reference
standards not be ground with the sample to be analyzed but
added and blended with the sample after it has been
ground.
E. Separation Methods
Often times in order to analyze a phase of interest or to
identify phases in low concentrations it is necessary to
physically or chemically treat a sample to concentrate the
material(s) of interest. After treatment, the analyst still must
consider the preparation methods outlined in these sections
before analyzing the sample. Separation methods are too
numerous and detailed to be discussed here, however,
general methods are outlined below. Separation methods
take advantage of physical or chemical differences among
materials.
Method
Purification by Crystallization
Sieving
Magnetic Separation
Microscopic Separation
Selective Filtration
Density Gradient
Chromatographic Methods
Membranes, Molecular Sieves
Electrochemical
Froth Flotation
Chemical/Physical Difference
Solubility
Particle Size and Habit
Magnetic Susceptibility
Color, Texture, Size, Habit
Settling Rate
Density
Partition Coefficients
Molecular Size and Transport
Electronic Structure
Particle Size, Density
F. Variable Temperature Methods
Both high and low temperature capabilities are common in
diffraction laboratories. Low temperatures can freeze
molecular motions producing high quality data and struc-
tural information on materials which may be liquid or
gaseous at room temperature. High temperature analyses
are often used to analyze industrial processes, particularly in
the polymer, catalyst, and ceramic areas. Samples are
usually heated or cooled from the back side of the sample, so
that the front surface will be open to the X-ray beam. This
configuration produces thermal gradients through the sam-
ple due to the heat loss from the sample surface.
A major problem of low temperature analysis is the
formation of ice on the cooled parts of the apparatus, in-
cluding the sample, during analysis. It is advisable to have
the cooled region in a vacuum or a chamber flushed with a
dry, inert gas such as He or N2. If an inert gas is used, this
gas can be cooled to provide the sample cooling. Several
methods outline procedures for controlling the flowing gas
temperature which is used to cool the sample. Heated gases
can be used to heat a sample. Alternatively a cover of mylar
supported over the sample may prevent ice buildup on the
sample surface. Studies43'44 have shown that calorimeters
can be designed to provide exact temperature control while
simultaneously measuring calorimetric and X-ray data.
Another problem in variable temperature work is that
when the temperatures induce a chemical change or phase
change in the sample, other changes may also take place.
Thermal gradients due to the heater or the orientation of the
sample holder substrate may induce orientation in the newly
formed phase. Some workers manufacture devices so that
the sample can be mechanically worked so as to reduce
orientation during analysis. The mechanical treatment may
also induce phase changes. Such treatments may be
necessary, so that the material can be identified.
Materials of construction are crucial in the develop-
ment of variable temperature apparatus. For example, Pt is
often used in high temperature analyses due to its resistance
to oxidation and lack of reactivity with the sample.
However, Pt can act as a catalyst and in some cases, at
temperatures above 400°C, Pt can alloy to other metals.
Materials must be chosen which are inert over the desired
temperature range of analysis. In inert atmospheres other
metals can be used as heater/sample supports including
graphite, tungsten, molybdenum, tantalum and rhenium5\
Again, sample reactivity is the limiting factor.
If small samples are being analyzed, it is desirable that
the sample holder and binders have the same thermal ex-
pansion (or contraction) characteristics as the sample. If the
materials have widely different thermal properties, strain
may be induced in the sample and the sample could also
change position during the experiment. A capillary is often
used to hold the sample in place. Rudman54 outlines several
methods used in preparing samples for variable temperature
analyses.
G. Common Sense Applications
Sections l.A-l.F outline many sample preparation pro-
cedures. However a few summarizing points may help the
analyst with sample preparation.
(a) Use of internal standards almost always helps
calibration and standardization of the data. Internal stan-
dards can be used to obtain accurate peak positions needed
for search/match procedures.
(b) If several sharp intense peaks are observed which
Powder Diffraction, Vol. 1, No. 2, June 1986 59
do not match the intensities in the JCPDS reference file for a
material, then the sample may be oriented and the analyst
should prepare the sample differently.
(c) Insufficient sample intensity can be an indication of
small crystallite size or high sample absorption. The analyst
may want to analyze a thinner sample to see if absorption is
a problem or a thicker sample which may place more
crystallites in the X-ray beam.
(d) Orientation can be detected by running the sample
in one position and then rotating the sample 90 degrees
about the diffractometer axis, and rerunning the sample. If
the diffracton intensities vary or if new diffraction peaks arise
and others are extinguished, then the sample is probably
oriented. If the sample is oriented in only one direction then
the sample may have to be moved 90 degrees again perpen-
dicular to the initial two directions.
Figure 6. Some common mounts for diffractometers. Mount 1 (Siemens) is a depression
mount with raised ridges for reducing orientation. Mount 2 is the 2-piece assembly of
Frevel (45) used as an adjustable height sample holder. A screw mechanism (2-a) raises
and lowers the glass insert (2-b). Mount 3 is a common depression mounting^ (Philips).
Mount 4 is a brass disk with the sample in the center which is used in Hagg-Guinier
cameras. The sample is dusted on a thin Alfilm substrate. Mounts 1, 2, and 4 are
designed for use in rotating sample holders.
2. Instrument Specific Methods
A. Methods of Sample Preparation For the Diffractometer
Figure 6 shows several of the more common sample holders
used in diffractometer analyses. All of these sample holders
use a depression or cavity in which to mount the sample. In
a diffractometer geometry the X-rays are diffracted from the
top of the sample. As mentioned in Section 1, the X-ray
penetration into the sample will depend on incident angle,
absorption and elemental composition of the sample. The
cavities are usually designed so the surface of the sample is
tangent to the diffractometer focusing circle. However, it is
not the surface of the sample which is critical, rather it is the
half depth of penetration of the X-ray beam in the sample
which most closely corresponds to the resultant peak posi-
tion. Very light element compounds, such as most polymers
or organics, allow considerable penetration of die X-ray
beam and concornmitant peak displacement and broaden-
ing. In such cases thick samples in a deep cavity mount may
be subject to more displacement errors than thin smears on
a glass slide. Frevel45 designed an adjustable height sample
holder so that the "effective" diffracting surface (i.e. theboundary at the half depth of penetration) could be made to
coincide with the parafocusing surface. The sample holder
consists of a support cylinder with a glass or quartz disk
which could be moved by a precise translation screw. The
sample could then be mounted on the disk surface.
Cavity mounts are commonly made of aluminum,
bronze, light metals, Bakelite, glass or leucite. Single
crystals, most commonly quartz or LiF, if cut and polished
correctly, may present a mounting surface which will not
diffract and give interfering peaks. Post46 suggests that
crystals of quartz or silicon be cut so that diffraction planes
originally parallel to the surface be inclined by one or two
degrees. The normal of these planes should be displaced in
the plane of incidence and diffraction from the planes will
generally not reach the detector in significant amounts. Post
also suggests that mounts not be made of heavy elements so
that fluorescent X-ray scattering can be minimized, reduc-
ing background noise.
Cavity mounts are most commonly either side loaded
or back loaded. A frosted or serrated glass surface, ceramic
or cardboard is placed over the front of the mount (to be ex-
posed to the X-ray beam), and the sample is carefully added
through the open back or a side port until the cavity is full.
Then the back or side port is covered and the front piece is
removed carefully so as not to disturb the surface. If the
holder can only be front mounted then it is advisable to
retouch the surface by lightly cutting grooves with a sharp
edge, rolling with a knurled surface or tamping with a ser-
rated disc47. This technique may leave a roughened surface
so it is advisable to adjust the sample height. Pressing the
surface smooth with a microscope slide is ill advised as it
enhances preferred orientation in the surface layers which
contribute heavily to the resultant peak intensities. If pre-
ferred orientation is a problem (Section 1 .B) then the sample
may be mixed with a poorly scattering filler. Powdered
glass, cork, starch, gelatin, gum arabic and tragacanth have
been used. Starch has a spherical shape and after picking up
particles clinging to its surface, can be randomly packed into
the cavity.
A systematic study of intensity variations with front,
back and side loading (Visser48) on a diffractometer revealed
that side loading was a better method of packing for samples
of gypsum, a platy material.
If microsamples are to be analyzed, sample support
and position become crucial variables. Single crystals either
cleaved or cut on or off Bragg planes have been effective
sample supports. Fluorite, calcite and MgO can be easily
cleaved. Si and quartz can be cut off Bragg planes and
chemically polished. Quartz can be polished mechanically
by grinding on a flat surface such as a glass plate, with 600
SiC followed by a 90 second etch in HF. This treatment ef-
fectively eliminates peaks from the damaged surface layers.
BT cut quartz oscillator plates make excellent substrates.
Once a substrate is chosen, the sample must be
centered in the X-ray beam. A small depression in the sup-
port may help locate the sample. A binocular microscope is
also a handy aid in centering small samples. Crystals can be
60
Powder Diffraction, Vol. 1, No. 2, June 1986
transferred to the support by using glass capillaries, metal
fibers or a handy set of dental tools (picks of different sizes
and curvatures). Once the sample is centered a binder or
lacquer can be used to hold it in place. Two sided tape can
also be used to hold the sample to the center of the support.
With small samples, beam scatter can become an important
consideration. If possible, the sample chamber and beam
path should be evacuated or alternately flushed with He.
Helium should be kept away from the high voltage of the
detector, as it has a low dielectric constant and allows elec-
trical breakdown.
Special sample preparation procedures may be
necessary for quantification on a diffractometer. Frevel and
Roth49 describe a procedure for quantitative semimicro
assay using a diffractometer. They describe a preparation
method which produces a thin powder layer on a conven-
tional rotating sample holder. The procedure experimental-
ly determines the mass absorption coefficient for the sample
being analyzed independent of the crystallinity of the sam-
ple. Absorption effects are minimized by using thin small
samples (5-10 mg) and correcting for absorption. The
authors note that this and other quantification methods are
sensitive to crystal perfection, crystallite size (Section l.A),
particle size (Section l.C) and the quality of any reference
standards used.
Figure 7. Mount 1 is a 2-piece O-nng assembly used for reactive samples. Samples are
placed in the depression cavity in a dry box and then sealed with a thin layer ojMylar us-
ing the 0-nng assembly. Mount 2 is a diffractometer mount usedjor analyzing thin films.
For slightly reactive samples, hygroscopic materials,
slurries or liquids, a sealed sample holder may be necessary.
A simple device is to construct a sample holder with an
O-ring seal as shown in Figure 7. The sample can be placed
in a depression in the holder, then sealed with a thin film of
plastic. Mylar is commonly used as the seal; care must be
taken that the diffraction pattern of the film does not in-
terfere with the diffraction pattern of the sample. It is ad-
visable to run a blank sample holder so that diffraction peaks
from the sample holder can be accounted for. An alternative
method is suggested by Prof. D. L. Wertz (U. of Southern
Mississippi) who uses the following three step procedure:
(1) Cover the window of the sample with 0.1 mil
Mylar and seal the window with hot polyethylene;
(2) Add the sample through an entry port (side or back
loading), or if a liquid, the sample can be added with a
syringe;
(3) Seal the entry port with polyethylene.
There are many methods used for analyzing clay
samples. As mentioned in Section IB, several of these
methods purposely orient the sample. If the sample is
oriented the diffraction pattern will match only the 00i
peaks of the JCPDS reference pattern with respect to peak
intensities. For specific identification, unoriented samples
should be run for complete identification of the clay.
Oriented samples can be used to identify clay mineral
families, and for quantitative analysis. Thomson, Duthie
and Wilson^ describe methods for producing randomly
oriented clay diffraction patterns, for both qualitative and
quantitative analyses. Spray drying methods^.26,51 are
generally excellent for producing randomly oriented
samples. The filter-membrane peel method28^0 is often
cited as a good technique for getting randomly oriented
samples. In this method particles are sized, suspended and
quickly filtered on a membrane. However, as Edmonds et
al.21 have shown, not all membranes are alike and some
membranes contain large pores which can orient a sample.
References 28, 30, 52 and 56 describe and compare
several common techniques used to orient clay minerals for
quantitative analyses. An advantage of orientation is that
sensitivity is usually increased.
B. Methods of Sample Preparation for Guinier Cameras
A common factor in Guinier instruments is that they use
focusing optics through the use of curved monochromators.
The most common optical arrangement is to have the film
cassette or counter in the transmission subtraction position
(Figure 8). This optical configuration eliminates the Ka2
component of the incident radiation and minimizes
chromatic dispersion effects. In this configuration the X-rays
are transmitted through the sample. As a result, most sam-
ple preparation methods use thin samples and lightly ab-
sorbing sample binders and substrates to minimize absorp-
tion effects (Section l.C).
A common method used by Brown and Foris42 is to
stretch 3M MagicTransparent Tape over a metal
diaphragm or O-ring, as shown in Figure 9. Then a
previously ground sample is dusted onto the tape. Some of
the authors use either a thin Mylar or Al film for a support
(once again on a metal diaphragm). Vaseline is then thinly
smeared onto the support and the sample is then dusted on-
to the Vaseline. If the sample is hygroscopic or if internal
standards are needed, then these may be blended with the
Vaseline and smeared on the support21.
Some Guinier cameras are equipped with goniometer
mounts so that small samples may be analyzed. Samples are
usually mounted on a small glass fiber using Vaseline or
Duco cement. As with single crystal methods used in
crystallography, the samples may also be encapsulated in
Powder Diffraction. Vol. 1. So. 2, June 1986 61
Long fine focus
X- Diffracted beam
Seeman-Bohlin
focusing cylinder
Line focus
of X-ray tube Curved
focusing
monochromator
Primary beam
focal line
Figure 8. Huber-Guiner high resolution diffraction system, transmission-subtraction
geometry.
Figure 9. Various mounts used for Huber-Guinier systems. Mount 2 shows the 2-piece
O-ring assembly while Mount 1 shows a mounted sample. The sample is dusted on a
thin Al substrate coated with Vaseline. Mount 3 shows an O-ring assembly with angle
markings used in the analysis of orientation in polymer films.
thin glass capillaries if they are sensitive to moisture or re-
quire a special atmospheric environment. Thin walled
capillaries are necessary to avoid absorption of the X-ray
beam.
For temperature dependent work a fine Pt-10% Rh
gauze of 80 mesh can be obtained from Englehard In-
dustries. The fine mesh has an average thickness of 76 ^tm.
Fine powders can be lightly pressed into the mesh-^ without
the need of binders. The Pt-10% Rh can also be used as an
internal reference material. A more conventional method is
to use a binder (such as Carbowax) and suitable solvent and
slurry the sample in the mixture. The mixture is then coated
onto a Pt sample loop. The loop can then be mounted on a
conventional goniometer. If a binder is used then care must
be taken that the binder will not crack or thermally change
during the experiment.
Because most Guinier experiments use a transmission
geometry, small thin samples must be used. A typical
Guinier experiment may use anywhere from a tiny single
crystal to 20 mg of sample. When small samples are used
the analyst must be careful to choose representative
materials and the analyst must avoid orientation (Section
l.B). Guinier cameras have a wide variety of sample rota-
tors. As mentioned in Section l.B these rotators do reduce
the effects of orientation but do not remove orientation. For
example, Huber-Guinier cameras have a sample oscillator
that only moves the sample back and forth in one direction.
Single crystal attachments usually rotate the sample around
the mount direction. On the other hand, Hagg-Guinier
cameras rotate the sample 360 degrees in the plane of the
sample but do not oscillate the sample in any other direc-
tion. If an oriented sample is used these different types of
Guinier cameras will give different I's in the diffraction
pattern.
Because focusing optics are used, sample position
(l.D) becomes crucial to the success of the experiment. It is
suggested that some type of internal standard be used to
calibrate the sample for position.
C. Methods of Sample Preparation For Debye-Scherrer
and Gandolfi Cameras
Many laboratories use Debye-Scherrer cameras. These
cameras are especially useful for applications involving small
samples or unusually reactive materials that can be loaded
into capillaries. The Gandolfi modification allows even
smaller samples to be used including single crystals.
Samples for the Debye camera are usually rod shaped and
may be 0.1-0.5 mm in diameter and up to 1 cm long,
although only about 0.5 mm of the length is seen by the
X-ray beam. These rods may be wires or fibers or a
powdered sample may be packed in a capillary or cemented
to the outside of a support fiber such as a thin glass rod.
Cements include those mentioned previously. Collodion in
ether seems to be one of the best selections as it dries rapidly.
Samples for the Gandolfi camera are balls of powder or
clusters of crystallites mounted on the tip of a fiber support.
Packing powder in a capillary can be a frustrating ex-
perience. Commercial capillaries are available with a funnel
on one end and in diameters of 0.1-2.0 mm. Usually a 0.3
mm size is satisfactory for most situations. After grinding the
powder to be sure that there are no coarse grains remaining
that could plug the bore of the capillary, a small amount of
powder is placed in the funnel. The funnel is tapped until
the powder moves down into the upper part of the capillary.
Using a match, microtorch, or hot wire, the funnel is cut off
and the capillary is sealed. A i m long glass tube with a 5-7
mm i.d. is placed upright on a table or other firm surface,
and the capillary is dropped down this tube narrow end first.
The momentum in the powder will tamp the powder toward
the end as the capillary bounces on the table surface. The
purpose of the tube is to confine the capillary for easy
recovery and repeated droppings. Several droppings will be
necessary to move all the powder to the narow end of the
62 Powder Diffraction, Vol. 1, Xo. 2,Junel9H6
capillary. Failure to remove the funnel will cause the
capillary to crush in the tamping procedure. Capillaries
made in this way may also be used in the diffractometer or
Guinier camera.
Samples for the Gandolfi camera should be mounted
on the tip of a fiber already mounted in the camera holder.
A convenient way to accomplish this mounting is to place a
drop of collodion in amyl acetate on a microscope slide and
to put the grain to be mounted into the drop. Using a
binocular microscope, the grain can be isolated and made to
adhere to the end of the fiber. For extremely tiny grains, the
drop is allowed to dry first. The glass fiber is sharpened by
touching it to a hot wire and drawing it away. A needle is
used to loosen the collodion film around the particle, and
this film is picked up on the sharpened fiber. Holding the
fiber over the surface of some amyl acetate will soften the
collodion and allow it to adhere to the glass fiber. The secret
for obtaining diffraction patterns from such tiny grains is
perfect centering of the grain in the camera and evacuation
of the camera chamber during the exposure. Grains as small
as 5/im in diameter may produce patterns.
Generally a selection of capillary tubes of different in-
ner diameters are useful so that a tube can be chosen which
will meet the dimensional requirements of the sample. In
addition, small samples require small capillaries so that the
scatter from the capillary does not swamp out the signal
from the sample. If the sample is non-reactive then mount-
ing the sample on top of the capillary or fiber using a scope
will minimize the effects of the support on the data.
An alternative method of mounting the sample is to
rub a glass capillary or fiber to electrostatically charge it,
then use the fiber to pick up the crystal(s) which are then
mounted inside the capillary. Reference 38 outlines several
methods for mounting and sealing capillaries for room
temperature, high and low temperature analyses.
The disadvantage of using small samples is that there
may not be enough randomly oriented crystallites in the
X-ray beam to produce a pattern without orientation. In
this case sample preparation (Section l.A and l.B) and a
sample rotator may be crucial to the success of the experi-
ment.
References
1. R. Jenkins, Adv. X-Ray Anal., 17,32 (1974).
2. J. H. Burns and M A. Bredig,/ Chan. Phys., 25, 1281 (1956).
3. R. Schrader and Br. Hoffrnan, Z. Chem., 6, 388 (1966).
4. J-L. Dandurand, C. R. Acad. Sci. Paris,Ser. D, 881 (1970).
5. S. J. Gregg, Chem. Ind., 11, 611 (1968).
6. D. O. Nothwood and D. Lewis, Amer. Miner., 53, 2089 (1968).
7. D. Lewis, D. O. Northwood and R. C. Reeve,/ Appl. Ctyst, 2, 156
(1969).
8. M. Senna and H. KunoJ. Am. Ceram. Soc., 54, 259 (1971).
9. F. DachilJe and R. Roy, Nature, 186, 34 (1960).
10. H. Takahashi, 6th Nat. Conf. Clays and Clay Minerals, Pergamon:
New York, Vol. 2, 279 (1959).
11. A Panis, C. R. Seance Acad. Sci. Paris, Ser. D, 1057 (1970).
12. L. Vegard and Th. Hauge, Z Physik, 42, 1 (1927).
13. J-L. Dandurand, C. R. Seance Acad. Sci. Paris, Ser. D, 808 (1970).
14. I. J. Lin, Israel J. Earth Sa., 20, 41 (1971).
15. T. G. Burton, Trans. Inst. Chem. Engrs., 44, 37 (1966).
16. J. M. Bloch, Bull. Soc. Chim. Fr., 774 (1950).
17. H. P. Klug and L. E. Alexander, X-Ray Diffraction Procedures, 2nd
Ed., John Wiley and Sons Inc., N.Y., N.Y. (1974).
18. W. Parrish and T. C. Huang, Accuracy and Precision of Intensities in
X-Ray Polycrystalline Diffraction, Adv. in X-Ray Analysis, 26, 35-44
(1983).
19. M. C. Morris H. F. McMurdie, E. H. Evans, B. Paretzkin, J. H.
deGroot, R. Newberry, C. R. Hubbard and S. Carmel (1977). Nad.
Bur. Stand. (U.S.) Monogr. 25, 14, 1-5.
20. M. C. Morris, H. F. McMurdie, E. H. Evans, B. Paretzkin, H. S.
Parker, N. P. Pyrros and C. R. Hubbard (1984). Nad. Bur. Stand.
(U.S.) Monogr. 25, 20, 1-5.
21. J. W. Edmonds, W. W. Henslee and R. E. Guerra, Anal. Chem., 49,
2196 (1977).
22. L. Segal, J. J. Creely, A. E. Martin Jr., and C. M. Conrad, Textile
Res. Journal, 786, Oct. (1959).
23. L. D. Calvert, A. F. Sirianni, G. J. Gainsford and C. R. Hubbard,
Adv. in X-Ray Analysis, 26, 105 (1983) and references dierein.
24. R. C. Rau, Norelco Reporter, Vol. X, No. 3, 114 (1963).
25. L. F. Vassamillet and H. W. King, Adv. in X-Ray Analysis, 6, 142
(1963).
26. F. D. Bloss, G. Frenzel and P. D. Robinson, Am. Mineral, 52, 1243
(1967).
27. L. E. Copeland and R. H. Bragg, Preparation of Samples for die
Geiger Counter Diffractometer, ASRM Bulletin No. 228 (1958).
28. R. J. Gibbs, Am. Mineral, 50, 741 (1965).
29. H. G. Shaw, Clay Mineral, 9, 349 (1972).
30. J. I. Drever, Am. Mineral, 58, 553 (1973).
31. L. S. Birks, M. Fatemi, J. V. Gilfrich and E. T.Johnson, "Quantita-
tive Analysis of Airborne Asbestos by X-Ray Diffraction", Naval Re-
search Laboratory NRL Report 7874 (1975).
32. M. Fatemi, E. T. Johnson, L. L. Widock, L. S. Birks and J. V. Gil-
frich, "X-Ray Analysis of Airborne Asbestos, Interim Report: Sample
Preparation. Environmental Protection Agency EPA-6O0/2-77-O62
(1976).
33. C. H. MacGillavary, G. D. Rieck and K. Lonsdale, Editors, Interna-
tional Tables for X-Ray Crystallography, Vol. Ill, Physical and
Chemical Tables, The Kynoch Press, Birmingham, England (1962).
34. F. H. Chang, J. Appl. Cryst., 7, 526 (1974).
35. B. L. Davis and L. R. Johnson, .4*. in X-Ray Analysis, 25, 295(1982).
36. W. N. Schreiner and R. Jenkins, Adv. in X-Ray Analysis, 25, 231
(1982).
37. T. C. Huang and W. Parrish, Adv. in X-Ray Analysis, 25, 213 (1982).
38. A. Brown, J. W. Edmonds and C. M. Foris, Adv. in X-Ray Analysis,
24, 111 (1981).
39. A. Brown, Adv. in X-Rqy Analysis, 26, 11 (1983).
40. W. Schreiner, T. G. Fawcett, Adv. in X-Ray Analysis, 28, 309 (1985).
41. National Bureau of Standards, Office of Standard Reference Materials,
Gaithersburg, MD 20899.
42. A. Brown and C. M. Foris, Adv. in X-Ray Analysis, 26, 53 (1983).
43. T. G. Fawcett, R. A. Newman, C. E. Crowder, W. C. Harris, L. F.
Whiting, F. J. Knoll, J. C. Tou, V. J. Caldecourt and W. E. Smidi,
Adv. in X-Ray Analysis, 28 (1985).
44. J. T. Koberstein, T. P. Russell, ACS Polymeric Materials Science and
Engineering Preprints, 51, 141 (1984).
45. L. K. Frevel, J. Appl. Cryst., 11 (1978).
46. B. Post — Laboratory Hints for Crystallographers.
47. D. K. Smith and C. S. Barrett, Adv. in X-Ray Analysis, 22, 1 (1978).
48. J. W. Visser, 1965 report to ASTM.
49. L. K. Frevel, W. C. Roth, Analytical Chemistry, 54, 677 (1982).
50. A. P. Thomson, D. M. L. Dutiiie and M. J. Wilson, Clay Minerals, 9,
345 (1972).
51. R. Hughs and B. Bohor, The American Mineralogist, 55, 1780 (1970).
52. P. J. Nel and R. E. Oberholster, Evaluation of Three Mounting
Techniques for die X-Ray Diffraction Analysis of Clay Minerals.
53. T. G. Fawcett, P. Moore Kirchhoff and R. A. Newman, Adv. in
X-Rqy Analysis, 26, 171 (1983).
54. Rudman, Low Temperature Diffraction, Chapter 6, p. 161.
55. B. D. Cullity, Elements of X-Ray Diffraction, 2nd Ed., Addison-
Wesley Pub., Reading, Mass. (1978).
56. P. L. Hauff, H. C. Starkey, P. D. Blackman and D. R. Pevear, U.S.
Geological Survey Open File Report 82-934, available via die Clay-
school, P.O. Box 1000, Conifer, Colorado 80433 - see references
dierein.
57. D. K. Smith, Noreko Reporter, 10, (1963).
Powder Diffraction, Vol. 1, No. 2, June 1986 63