by H J VAN HOOK

Phase Relations in the Ternary System Fe,O,- FeO-YFeO, by H. J. VAN HOOK Research Division, Raytheon Company, Walthom, Mossachuset,s Equilibrium data are presented for the ternary system Fe203-Fe0-YFe03 in ambient atmos- pheres of air, oxygen, and carbon dioxide at melt- ing temperatures. The temperature range for the coexistence of yttrium-iron garnet and oxide liquid decreases with decreasing oxygen partial pressure from 127°C in oxygen to 86°C in air to 28°C in carbon dioxide. In addition, the com- position of the garnet phase crystallizing from these melts changes with variations of tempera- ture and oxygen pressure. The experimental data are discussed in terms of a polythermal- polybaric model of the system. Some conclu- sions are drawn as to the limiting conditions in different atmospheres for the growth and the re- sulting composition of yttrium-iron garnet crystals in equilibrium with ternary oxide liquids. 1. Introduction iIr3 growth of single crystals of oxide materials has been the object of considerable interest in recent research from T both practical and theoretical viewpoints. The interest arises from the unusual magnetic and electrical properties of some of these oxides and from the fact that these properties can be investigated most readily and unambiguously on single crystals. To prepare crystals reproducibly and to predict the dependence of stoichioinetry on growth conditions, a pre- vious knowledge of the phase equilibria in the melting range is desirable. Equilibrium data are critically important in systems in which decomposition occurs within the tenipera- ture-composition range of interest and where the gaseous phase must be considered as an additional variable. This situation commonly arises in high-temperature studies of the transition metal oxides as shown by Muan’ and by others2 and is true, in particular, of iron oxide, which is one of the components studied in the present work. Since the discovery of I‘errirnagneti~m~ in the garnet-type oxides, single crystals of yttrium-iron garnet have been syn- tliesizcd by Nielscn and Dearl)orn4 using a lead oxide flux and more recently by tlic Verneui15 and zone meltingG tech- niques. Interest has also developed in this laboratory in the alternative possibility of growth directly from a melt of the constituent oxides to produce garnet crystals with a mini- mum of crystalline imperfections. The system yttriun-iron- oxygen was chosen for study to cvaluate the problems in growth in a system which might be considered to be repre- sentative of this entire class of oxides. Phase relations in this system in air have been reported previously7; the present paper combines these results with data obtained in two other oxygen atmospheres to givc some indication of relations in the ternary system as a whole. II. Experimental Procedure The experimental methods used in this study have been described previously’ and will be dealt with very briefly here. The starting materials were copr‘ecipitated yttrium-iron hydroxides analyzed separately for yttrium and iron to 3~0.5 mole %. Individual samples were packed in open platinum- rhodiutn vials and suspended within a platinum-rhodium- wound resistance furnace for varying periods of time before quenching. Temperatures in the range 1400” to 1650OC were measured by a 95Pt5Rh vs. 80Pt20Rh thermocouple calibrated at the melting points of CaMgSizOF (1391.5”C) and CaSiOa (1544°C) as defined by the Geophysical Laboratory tcm- perature scale.* The thermocouple was placed adjacent to the samples during each run. Quenching was effected by dropping the sample plus vial into a water bath at the bottom of the furnace or by slower cooling in a stream of the gas used in the specific experiment. Ambient atmospheres other than air were achieved by a continuous flow (1.5 liters per hour) of the desired gas through the enclosed tube of the furnace and out the top through a glass capillary. The gases used were 99.9% pure oxygen, air, and carbon dioxide. Im- purities were negligible except for 0.05% oxygen in the CO2 which was removed by passing the C02 gas over copper gauze at 500°C. The phases present at melting temperatures were deter- mined after quenching by microscopic study and by X-ray diffraction techniques to identify the crystalline and quenched liquid phases present at the temperature of equilibration. Since the Fe2O3-Y2O3 starting compositions lose oxygen in the temperature range of interest, a separate study was made by thermogravimetric analysis of weight changes in the different oxygen atmospheres for seven critical compositions. The compositions studied were 0, 2.5, 7, 13.5, 22, 35, and 38% YzOa in mole ratio of Y203-Fe203. The extent of oxygen loss in these materials is represented by position along lines of constant Y/Fe ratio extending into the ternary system away from the oxygen apex. These oxygen reaction lines are Receivcd June 26, 1961; revised copy received Octobcr 2, 1961. This work was supported in part by the Electronics Research Directorate, Air Force Cambridge Research Laboratories, Office of Aerospace Research, United States Air Force, Contract No. The writer is a research staff member, Solid State Physics Group, Research Division, Iiaytheon Company. Arnulf Muan, “Phase Equilibria at High Temperatures in Oxide Systems Involving Changes in Oxidation States,” Am. K. G. Richards and J. White, “Phase Relationships of Iron-Oxidc-Containing Spincls: I, Relationships in the System Fe-Al-0,” Trans. Brit. Ceuam. Soc., 53 [4] 233-70 (1954); Ceram. Abstr., 1960, July, p. 176a. (b) A. E. Paladino, “Phase Equilibria in Ferrite Region of the System FcO-MgO-FezOa,” J. Am. Ceiam. Soc., 43 [4] 183- 91 (1960). Felix Bertaut and Francis Forrat, “Structure des Ferrites FerritnagnCtiques des Terres Rares” (Structure of the Ferri- rnagnetic Rare Earth Ferrites), Compt. rend., 242 [3] 382-84 (1956). J. W. Nielsen and E. F. Dearborn, “Growth of Single Crys- tals of Magnetic Garnets,” Phys. and Chem. Solids, 5 [3] 202-207 (1958). K. G. Rudness and R. W. Kebler, “Growth of Single Crys- tals of Incongruently Melting Yttrium Iron Garnet by Flame Fusion Process,” J. Am. Ceram. Soc., 43 [l] 17-22 (1960). L. L. Abernethy, T. H. Ramsey, Jr,, and J. W. Ross, “Growth of Yttrium Iron Garnet Single Crystals by Floating Zone Tech- nique,” J. Appl. Phys., 32, Suppl. No. 3, 376-779 (1961). ’I H. J. Van Hook, “Phase Relations in the System Fe&- FeB04-YFeOs in Air,” J. A%. Ceram. SUC., 44 [5] 208-14 (1961): 8 R. B. Sosman, “Temperature Scales and Silicate Research,” Am. J. Sci., Bowen Volume, Part 11, 517-28 (1952); Ceram. Abstr., 1957, October, p. 259h. AF 19(604)-5511. J. SC~., 256 [3] 171-207 (1958). 2 (a) 162 April 1962 Phase lielations in the Ternary System Fez03-FeO- YFeO, 163 I' I I /' Orthoferrite old IIQJI~ 1600C - . .. .. I Garnet and lhqu~d : . .. ,. .. 1 Orthoferrite aid I goiriet Magnet le ond gorntt J -j 1:. ; . * ..I 1401) .." . .. IBi"? 30 Hemot le and gornet _L - 1- I Y203 [mole Yo) Irol OldL 10 10 50 40 YFeOj Fig. 1. Phase relations in the system FesOs-FeO-YFeOl in air. Lower diagram shows phases present as a function of temperature and initial composition; triangular diagram shows ternary composition of the liquids (dash double-dot fine) and of the crystalline garnet and magnetite phases (stippled oreos). Light lines with orrows in the triangular plot are oxygen reaction lines; the isothermal reaction points with temperatures indicated are discussed in the text. shown in the three isobaric phase diagrams with arrows indi- cating the trend of total composition resulting from oxygen 10% Using a thermobalance and samples weighing 10 g contained in platinum cruciblc.;, a weight loss or gain of O.OOlyo could be detected readily. The accuracy ol weight nieasurcment, howevcr, was limited because of loss in weight of the platinum crucible through volatilization which reached significant levels above 1400°C. At 1.500"C an empty crucible lost g per cm2 of surface area per hour in still air or approximately 0 (55 mg per day The weight loss was approximately twice this value in a flow (I 5 liters per hour) of air through the furnace tube. This level of weight loss was not significant in samples high in iron oxide where relatively large weight changes (1 to 2 wt%) were recorded and where equilibrium could be reached at each temperature in an hour or less. TTowever, in samples near the garnet composition, i e , 35 and 38Y0 Y203, weight changes were the order of 0.1 wt% (10 mg) and were attained very slowly even above 1500°C. The accuracy of direct therniobalance measurements near the garnet composition therefore is limited to -f I mole yo FeO or a value of x = 0 005 in (Y, Fe)z03-,. This is approxi- mately one half the precision in determinations of yttrium and iron content in starting compositions by chemical analysis 111. Experimental Results ( 11 The Air Isobar Equilibrium relations in air are shown in Fig. 1, which suin- marizes the data obtained from the quenching study (lower diagram) and from measurements of weight loss and gain (upper diagram). The lower figure is a projection of the ternary diagram and indicates phase changes in terms of starting mixtures of Fe203 and Y203 independent of the oxygen / I /' I Orthoferrite and I qu d Hemotte and garnet 3 Y203 (mole %) Fig. 2. Phase relations in the system Fe203-FeO-YFe03 in oxygen. Relation of upper to lower diagram is the same as in Fig. 1, with the oxygen isobar (dash double-dof fine) giving liquid compositions, lines with arrows as oxygen reaction lines, and isothermal reaction points labeled with appropri- ate temperatures. loss which has taken place at these temperatures. The ternary composition of the condensed liquid and crystalline phases is shown above in mole percent of the components FezOs, FeO, and YFeO3. In air there are three reaction temperatures involving three condensed phases and an atmos- phere with a partial oxygen pressure of 159 mm Hg. These points appear as horizontal lines in the projection and as three phase triangles in the ternary plot. With increasing tem- perature the first reaction point in air occurs at 1387°C in- volving hematite, magnetite, and yttrium-iron garnet, Yo.740- Fel.~soO~,gso. The second reaction point having the appear- ance of a eutectic point in the projected binary diagram occurs at 1469°C between magnetite, garnet, YO.~~SF~~.~~~OZ.~~~, and oxide liquid, Yo.27FeL.7302.87. Yttrium-iron garnet decomposes in air at 1555°C giving rise to a third reaction point with yttrium orthoferrite, garnet, Y0.74~Fel.~500~.960, and liquid composition, Yo.44Fel .5602.88. (2) The Oxygen Isobar A comparison of Fig. 2 with the previous figure shows that a change in oxygen pressure from 159 to 760 mm Hg has a marked effect on liquidus relations in this system. The hema- tite-magnetite transition has been raised by increased oxygen pressure to a point just above the minimum melting tempera- ture, resulting in a small primary crystallization field for the hematite phase. The minimum melting temperature is 1455°C and the decomposition temperature of yttrium-iron garnet is 1582°C. Thus, the temperature range for the stable coexistence of garnet and an oxide liquid is extended to 127°C in oxygen as compared with S6"C in air. Three reaction points involving two crystalline phases, a liquid, and oxygen gas (Fig. 2) are as follows: hematite + garnet, Y0.74r Fel.z500z.975 + liquid, Yo.zsFe~.~60~.90 at 1455OC; hematite + magnetite + liquid, Yo.&e1.78c2.89 at 1459°C; and ortho- ferrite + garnet, Y0.745Fe1.25002.94~ + liquid Yo.46Fe1.5402.88 at 1582°C. 164 Table I. Journal of The American Ceramic Society- Van Hook Lattice Parameters of Yttrium-Iron Garnet Solid Solutions Oxygen Air coz _-h_-- _-_- h--_ Temp Excess Excess Excess Excess Excess Excess (OC) FezOa Yzoa l‘eroa Yz03 Fez03 Yzos 1400 12.374 12.380 12.374 12.379 12.378 12.381 1440 12.375 12.381 14170 12.375* 12.381 12.37Cit 12.381 12.3811 12.382 1525 12.378 12.383 12.378 12.383 1540 12.380 12.383 Although melting relations are changed appreciably by changes in oxygen pressure, no measurable differences could bc found in composition of the garnet crystallizing from liquids in air or oxygen ambients Lattice parameter studies (Table I) and measurements of loss in weight indicate that solubility of excess iron oxide and loss of oxygen are essentially the same in hoth atmospheres within experimental error. (3) The C02 Section ’The oxygen partial pressure resulting from the dissociation of carl)on dioxide gas varies according to temperatureg and hcnce cannot be considered to be isobaric. In the tempera- ture range of interest in Fig. 3, the partial oxygen pressure increases from 0.96 mni IIg at 1450OC to 2.14 mm Hg at 1 (iOO°C The temperature range for the stable coexistence of garnet and liquid has been reduced considerably in a Con atmosphere as shown in Fig. 3. Significant changes have also taken place in the oxygen content of both liquid and crystalline phases. Two invariant situations involving the garnet phase arise: at 1467°C and Poz = 1.0 mm I-Ig with magnetite, garnet, Y0.74~Fe1.24~02.945, and liquid, YO.~~F~O.~O- 02.70, and at 1495°C and Poz = 1.3 mm Hg with orthoferrite, garnet, Yo .70cFel ~~0~. 9 t5, and liquid, YO.&el .s602 .78. Although the garnet phase is oxygen deficient at melting temperatures, the lattice parameter data of samples with iron oxide and with yttria in excess (Table I) indicate very little deviation from stoichioinetry in the metal atom ratio of 3/5. IV. Discussion of Results ( 1) Liquidus Relations The experimental results on melting relations in the three oxygen pressure sections cited in the foregoing demonstrate the controlling effect of oxygen atmosphere in this system. The reason for this will be apparent from a study of the ternary system as a whole. Figure 4 summarizes the data in the form of a ternary diagram with oxygen isobars drawn on the liquidus surface. The intersection of each isobar (dush double-dot lines) with the liquidus surface fixes the com- position of the melt in equilibrium with crystals as well as the temperature and composition for the intersection with the ternary boundary curves (heavy solid lines). The direc- tion of decreasing temperature along the boundary curves is shown by arrows. A temperature maximum exists along the iiiajinetite-garnet-liquid boundary curve at the point of inter- section with the magnetite-garnet join. This maximum temperature was not determined directly, but is known to be above 1469°C. The boundary curves separating the primary crystalliza- tion fields present a unique situation since four phases are present and, for a given oxygen partial pressure, conditions are invariant. The intersections with boundary curves give rise to the eutectic and peritectic reaction points, i.e., points L. S. Kassel, “Thermodynamic Functions of Nitrous Oxide and Carbon Dioxide,” J. Am. Clzenz. SOG., 56 [9] 1838-42 (1934). Vol. 45, No. 4 Magnetite .. * Gornet ond lhquld 1467*f4e E’ Magnetite and garnet 14001 Orthoferrite garnet 1 and Iran oxide 10 20 30 40 YFe0, Y203 (mole %) Fig. 3. not strictly isobaric as explained in the text. Phase relations in the system Fes03-FeO-YFeOs in COz. Section is Symbols and lines are the same as in previous figures. A through G in Figs. 1, 2, and 3, for each isobar shown in the preceding diagrams. It follows that there is a unique oxygen isobar that passes directly through the ternary eutectic U and another through the peritectic point V. These two points represent true invariant equilibrium with five coexist- ,FeO Fig. 4. Diagram summarizing relations in the system Fez03-FeO-YFeOa as deduced from three isobaric sections. Heavy lines are boundary curves separating primary crystallization fields, the dash double-dot lines indicate intersection of the isobars with liquidus surface, and light lines indicate liquidus isotherms. Points U and V are ternary eutectic and peritectic points respectively. Stippled oreas are crystalline single-phase areas at liquidus temperatures. Dashed lines represent inferred relations. April 1962 Phase Relations in the Ternary System FezOa-FeO- YFeOa 165 ing phases (gas, liquid, and three crystalline phases) in con- trast to the four-phase reaction points cited in the foregoing. The ternary invariant points U and TI are important because they define the critical limits for the stable coexistence of the yttrium-iron garnet phase and liquid; point U determines the minimum liquidus temperature and point V the minimum oxygen partial pressure. The ternary eutectic occurs at approximately 1453°C and 680 inm Hg oxygen pressure and the peritectic at an estimated 1465°C in an oxygen at- mosphere somewhat below that given by COz dissociation at that temperature, approximately 0.07 min Hg. The tein- peratures and oxygen pressures at these critical points can be estimated most readily from a plot of the four-phase boundary curves as a function of log goz vs. T. The equilibrium relations shown in Fig. 4 can be used to predict the situation at oxygen pressures higher and lower than those studied in this work. At pressures slightly lower than go, = 0.07 inin Hg the garnet phase will decompose on heating to a mixture of magnetite and yttrium orthoferrite below the liquidus, and the two latter phases will present a simple eutectic situation at the melting temperature in the isobaric projection. This means that the isobars will inter- sect the orthoferrite plus magnetite plus liquid boundary curve at some point above V in Fig. 4. At oxygen pressures exceeding 680 mm IIg, the isobar in question will intersect the ternary boundary curve GU at some temperature above 1453°C and HV above 1575°C. Both the minimum melting temperature and the decomposition temperature of the garnet will be increased by higher pressures owing to intersection of isobars with the boundary curves which are ascending in temperature as the fully oxidized Fe2O3-YI;eO3 sideline is ap- proached. There is some uncertainty as to whether the peritectic boundary curve (HV) will change in direction toward YFe03, possibly leading to the congruent melting of yttrium-iron garnet at very high pressures, although this does not appear to be likely from the projected trend of the curve. At any rate, this situation is experimentally inaccessible at present since very high temperature.; (>lGOO"C) and high oxygen pressures would be required. High pressures of oxygen gas are likely to increase the temperature range for coexisting garnet plus liquid because of very rapidly increasing tem- peratures along the boundary curve HV. (2) Garnet Composition The garnet phase in equilibrium with oxide liquid deviates from the ideal formula composition Y3Fe5012 with changes in temperature and in oxygen pressure. The small excess of iron in melt-grown yttrium-iron garnet is not surprising in view of the fact that the coexisting liquid always is iron-rich. Oxygen deficiency in these crystals might also be expected on the basis of the high-temperature conditions favoring the formation of some divalent iron in a compound originally con- taining only trivalent iron. The extent of iron excess in yttriuin-iron garnet was deter- mined by lattice parameter studies on a series of compositions fired in air at various temperatures and water-quenched.? Similar data have been developed for the oxygen and COZ isobaric sections (Table I). In each case the solubility limit was taken as the extrapolation of lattice parameter variation with composition in single-phase yttrium-iron garnet to the constant value of saturated yttrium-iron garnet in the pres- ence of a second phase. At constant temperature the lattice parameter change was 0.001 A per 0.1 mole yo Fe203 excess in starting mixtures of Fe203 and Y203. The X-ray data indicate that the solubility of iron is the same within experi- mental error (ItO.001 A) in air and in an atmosphere of pure oxygen gas. In an oxygen atmosphere given by COS de- composition, the iron excess in yttrium-iron garnet was only slightly above the limits of detection at all temperatures up to the decomposition point. The extent of oxygen deficiency in yttrium-iron garnet has been more difficult to establish experimentally because oxygen content at high temperatures cannot be controlled in the initial sample preparation as can be done with the metal atom content. Indirectly, variations in oxygen content of iron or of yttrium-saturated garnets could be correlated with lattice parameter changes as dependent on firing temperature and atmosphere At constant oxygen partial pressure, the lattice parameters of all quenched garnet compositions increase with higher firing temperature. At constant firing temperature the same compositions showed an increase in lattice parameter with decreasing partial oxygen pressure. Since both high temperature and low oxygen partial pressure generally favor the reduction of oxide materials, the similar effect of these variables on the lattice parameter of yttrium-iron garnet was taken as indirect evidence for reduction of the garnet. The variation in lattice parameter (Table I) is assumed to be related to the partial replacement of Fe3+ ions with larger Fez+ ions in the structure. Although the lattice parameter data definitely indicate changes in oxygen stoichiometry, the direct measurement of weight loss using a thermobalance proved to be rather difficult. Weight-loss measurements for yttrium-iron garnet are com- plicated by the very slow oxidation and reduction, by the gradual volatilization loss of the platinum crucible, and by the magnitude of the weight loss itself (= 0.1 wtyo). The results verify the trend in oxygen stoichiometry predicted by lattice parameter studies but were not accurate enough for quantitative measurements. A more indirect measure of oxygen stoichiometry was ob- tained by extrapolation of isothermal tie lines between the garnet and magnetite phases at 1387" and 1469°C in air (Fig. 1). Other extrapolations were made between garnet and oxide liquid in air at 1469°C (Fig. I) and in oxygen at 1455OC (Fig. 2) using the starting conlposition of 22 mole yo Y203. The composition heated above the liquidus and cooled slowly to near the minimum melting temperature in either air or oxygen atmosphere consists of 357' garnet and 65y0 liquid. Assuming that the garnet phase formed through crystallization has an equilibrium oxygen content, a tie line was drawn from the known composition to the minimum melting liquid (point A, Fig. 1; point D, Fig. 2) through the ternary composition of the 22% YzOa sample given by weight loss to an intersection with the garnet solubility surface. Although the extrapolation of tie lines is valid, the ac- curacy of the oxygen content determination is probably no greater than that of the direct thermobalance measurements near the yttrium-iron garnet composition described in the foregoing. It is hoped that oxidation studies on garnet crys- tals pulled from a melt at specific temperatures will give more definite information on oxygen content. It seems very likely that properties other than lattice parameters, e.g. re- sistivity, will also show dependence on oxygen content, lend- ing further support to the evidence for nonstoichiometry. From these observations one can deduce that the garnet solubility surface at liquidus temperatures extends into the ternary system through loss of oxygen and that the field width narrows through decreasing solubility of excess metal oxide as shown in Fig. 4. At low oxygen pressures the garnet phase is essentially stoichiometric in metal ratio but is oxygen de- ficient, whereas at higher pressures oxygen loss is lessened but solubility of iron is increased (Table I). These deviations from stoichiometry are unavoidable during equilibrium growth from an oxide liquid and may have important effects on the mechanism of the growth of the garnet crystals as well as on their physical properties. Acknowledgment The writer would like to acknowledge the assistance of C. R. Snider, M. E. Nichols, B. D. Roiter, and R. B. Hawkes in gather- ing the experimental data and E. A. Maguire and A. E. Paladin0 for criticism and discussion of the results.

by H J VAN HOOK

UFPE

Física

Continue navegando

Materiais relacionados

Perguntas relacionadas

Selecione a opção em que o capitalization foi incorretamente ut Poe crashed his Volkswagen van in an accident when tur Laura used to go to Hooki, a...

Quem foi o inventor do microscópio simples, também conhecido como microscópio de lente única?? Robert Hooke Zacharias Janssen Antonie van Leeuwe...

What are the topics covered in Module 4 of the course created by Felipe Torres? a) Hooks - Aplicação em Proteções, Hooks - Implementação 1, Hooks...

Antonie van Leeuwenhoek e Robert Hooke trouxeram contribuições significativas para o desenvolvimento da biologia, usando microscópios ópticos. Leeu...

Outros materiais