Phase relations in the system KFCeF3

Phase relations in the system KFCeF3

J. inorg, nucl. Chem., 197 I. Vol. 33, pp. 325 to 335. Pergamon Press. Printed in Great Britain PHASE RELATIONS IN THE SYSTEM KF-CeF3*¢ C H A R L E...

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J. inorg, nucl. Chem., 197 I. Vol. 33, pp. 325 to 335.

Pergamon Press.

Printed in Great Britain

PHASE RELATIONS IN THE SYSTEM KF-CeF3*¢ C H A R L E S J. BARTON, L. O. G I L P A T R I C K , G E O R G E D. B R U N T O N D A V I D HSU~: and H E R B E R T INSLEY§ Reactor Chemistry Division, Oak Ridge National Laboratory, P. O. Box X, Oak Ridge, Tenn. 37830 (First received21 April 1970; in revised form 20July 1970) A b s t r a c t - T h e system KF-CeF3 was investigated as a function of temperature and composition

by thermal analysis, differential thermal analysis, and gradient quenching. Crystalline phases in the quenched samples were identified principally by use of the polarizing microscope, but selected samples were examined by X-ray powder diffraction. Three compounds were identified: 3KF.CeF3, KF'CeF3, and KF.2CeF3 which melt incongruently at 675-+ 10°C, 755±5°C, and 1135± 15°C, respectively. Of these only KF'CeF3 is stable below 585°C. Three peritectic equilibria were observed corresponding to these incongruent melting points at 24, 32, and 63 mole % CeF3. The aKF-2CeF:~ phase exhibits a substantial solid solubility for KF at temperatures above 795°C which results in a region of cubic solid solution whose properties vary over the composition range from 50 to 66-2•3 mole % CeE:~. INTRODUCTION

PREVIOUS studies[l, 2] showed that phase relations in the LiF-CeF3 and NaFCeF3 systems are very similar to those of their LiF-PuF3 and NaF-PuF3 counterparts. The KF-CeF3 system was examined in order to show probable behavior for KF-PuF3. Puschin and Baskow[3] examined KF-CeF3 compositions containing less than 30 mole % of CeF3 and reported that the eutectic composition at 25 mole % of CeF3 melted at 660°C. Zacharaisen [4, 5] described a cubic high-temperature form of KF-LaF3 and KF-CeF3 and stated that the low temperature, form of these compounds is hexagonal. Schmutz [6] presented a complete phase diagram for the system KF-NdF~ and a partial diagram for KF-ErF3. He also reported X-ray data on the compounds KF.CeF3 and KF.2CF3, and indicated that 3KF-CeF~ is isostructural with the corresponding LaF3, PrF3, and NdF3 compounds. Brunton [7] has recently described the structure of/3-KF.CeF3, the low*Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. tPaper presented before the Inorganic Division at the 158th National Meeting of the American Chemical Society, New York City (September 1969). :~Summer employee from the University of California, Berkeley, now at Harvard University. §Consultant. 1. C.J. Barton and R. A. Strehlow, J. inorg, nucl. Chem. 18.143 ( 1961). 2. C.J. Barton, J. D. Redman and R. A. Strehlow, J. inorg, nucl. Chem. 20, 45 ( 1961 ). 3. N. Puschin and A. Baskow, Z. anorg, allg. Chem. 81, 361 ( 1913). 4. W. H. Zacharaisen, J. Am. chem. Soc. 70, 2147 (1948). 5. W. H. Zachariasen, Acta crystallogr. 2, 388 (1949). 6. H. Schmutz (Thesis), Investigations in the Alkali Fluoride-Lanthanide or Actinide Fluoride Systems, Kernreaktor Bau-und Betriebs-Gesellschaft m.b.H., Karlsruhe, Germany, KFK-431 (July 1966). 7. G. Brunton, A cta crystallogr. 25B, 600 (1969). 325

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C.J. BARTON et al.

temperature (orthorhombic) form of the 1:1 compound. Russian investigators [8] have also reported phase studies of the KF-NdFa system. Dergunov[9, 10] reported partial phase diagrams for several KF-rare earth trifluoride systems including LaF3, PrF~, SmF3, EuF3, and YF3. He found incongruently melting 1 : 1 compounds in the first two systems and congruently melting 3 : 1 compounds in the last three. The latter system was examined more recently by Porter et al. [11] and by Bukhalova and Babaeva [12]. Some discrepancies between the results of the last two investigations were noted [ 11]. These Russian investigators have also reported a partial diagram of the KF-CeF3 system[13] and one for the KF-ScF3 system[14]. Besse and Capestan[15] reported the preparation and X-ray properties of 3 KF.CeF3 and similar compounds. EXPERIMENTAL Our investigation of the KF-CeF3 system proceeded in two distinct stages. In the first stage, thermal analysis data were obtained with mixtures containing 5-50 mole % CeF~. Calculated weights of KF and CeF:~ (to give a 3-4 g total for each composition prepared) were placed in a 15 ml platinum crucible and blended with several grams of ammonium bifluoride in the closed nickel and stainless steel container shown in Fig. 1. A bare platinum-platinum 10 per cent rhodium thermocouple junction was positioned 1-2 mm above the bottom of the crucible. The mixture was then pretreated in situ prior to taking data by heating slowly in a flowing stream of purified helium until the ammonium bifluoride first melted, then decomposed. Finally the temperature was raised more rapidly until a temperature well above the estimated liquidus was attained (900°C). Pretreatment in this way produces a partial pressure of H F (Equation (1)) which reacts with any oxygen bearing species such as CeOF, KF.2H~O, or KOH which may be present as impurities to produce HzO (Equations (2) and (3)). By the time the maximum temperature has been reached all the NHaHF2 and oxygen species have passed from the equipment via the He stream leaving behind the nonvolatile system members in a homogeneous pure liquid state suitable for measurement. NH4HF..,

200oc , NH3 I' + 2 H F I'

(1)

Ce..,O3 + 6HF ~ 2CeF:~ + 3H20 1"

(2)

K O H + H F ~ K F + 2 H e O 1'.

(3)

Duplicate cooling curves were then recorded for each composition. At the termination of the thermal analysis measurements each composition was removed from the apparatus in a helium-filled box which was continuously monitored for water content and was kept near 1, but in no case greater than 2, parts per million. Gradient quenches were performed with ground portions of the recovered material in the manner previously described[16-18] and the resulting samples were examined by use of a polarizing microscope. Phases were identified from their optical properties. 8. B. S. Zakharova, L. P. Reshetnikova and A. V. Novoselova, Ves. M o s k . Univ., Ser. I1 22, 102 (1967). 9. E. P. Dergunov, Dokl. Akad. N a u k . S S S R 60, 1185 (1948). 10. E. P. Dergunov, Dokl. Akad. N a u k . S S S R 85, 1025 (1952). 11. B. Porter, R. E. Mealcer and P. R. Bremmer, Report BM-RI-7246 (March 1969). 12. G. A. Bukhalova and E. P. Babaeva, J. inorg. Chem. U S S R 11,644 (1966). 13. Ref.[12]. p. 624. 14. Ref. [12], p. 1959. 15. J. P. Besse and M. Capestan, C. r. hebd. SOancAcad. Sci. Paris 266C, 551 (1968). 16. P. A. Tucker and E. F. Joy, A m . Ceram. Soc. Bull. 36, 52 (1957). 17. C.J. B a r t o n e t a l . , J . A m . Ceram. Soc. 41, 63 (1958). 18. H . A . Friedman,Am. Ceram. Soc. Bull. 42, 284 (1959).

Phase relations in tile system KF-CelF:~

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328

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Compositions containing 50-95 mole % CeFa were explored in the second part of the study after a period of time during which an improved differential thermal analysis (D.T.A.) unit was developed. This unit which is described elsewhere[19] was used in conjunction with the gradient quench technique to examine 10 g preparations. These were purified by a modified NH4HF2 method. Mixtures with calculated compositions were placed in separate graphite thimbles and heated together in a nickel chamber. Six samples were prepared simultaneously followed by grinding and manipulation in the dry box. All D.T.A. cells and quench tubes in this latter series of compositions were evacuated at 100°C and sealed by welding at a vacuum of 5 × 10-5 torr or better prior to use. This procedure was necessitated, in part, by the hygroscopic nature of KF. Experimental compositions are listed in table 2 and represented as ticks at the base of Fig. 2.

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Fig. 2. The system KF-CeF3. 19. L. O. Gilpatrick, S. Cantor and C. J. Barton, Proc. 2nd Int. Conf. Therm. Anal., Worcester, Mass., August 1968 (Edited by Robert F. Schwenker, Jr. and Paul D. Garn), Vol. 1, p. 85. Academic Press, N e w York (1969).

Phase relations in the system KF-CeF3 RESULTS

AND

329

DISCUSSION

The equilibrium phase diagram for the KF-CeF3 system shown in Fig. 2 is based on thermal analysis and D T A data found in Table 1 and quenching data given in Table 2. Where the thermal data and quenching data were not in agreement, we relied mainly on the quenching data because equilibrium was not quickly achieved in this system, especially for sub-solidus reactions, and thermal analysis is a dynamic process. An exception to this rule was noted for compositions in the KF primary phase field where thermal analysis gave higher, and apparently more reliable, liquidus temperatures than were obtained by use of the quenching technique. Most of the thermal effects in Table 1 can be related to phase changes shown in Fig. 2, with due allowance for undercooling in the thermal analysis (TA) data. The effects noted in the range 485-500°C with compositions containing 5 - 4 0 mole % CeF3 are due to delayed decomposition of the 3KF-CeF3, KF'CeF3, and KF.2CeF3 but of these KF.CeF is the only compound stable below 585°C. Table 4 lists optical p(operties and other data for the phases in this system. The 3KF-CeF3 compound decomposes on heating at 675°C giving KF.CeF:~ and liquid. KF.CeF:~ decomposes on heating into a cubic solid solution plus liquid at 755°C in the composition range of 32-50 mole % CeF3. Quenched compositions at a fixed temperature such as 760°C in the region marked "Cubic Table 1. Thermal analysis (TA) and differential thermal analysis (DTA) data for the system KF-CeF3 Composition (mole % CeF:0 5

10 15 20 :22 25 127 30 :35 40 45 .50 52 55 .58 61 64"3 66'7 '72 80 82

Liquidus

Eutectic

809 780 686 633 622" 651 666 692

619 620 622 620 619 622 611

951 1069

Thermal effects (°C) Peritectic Solid transition 486* 490* 495" 485" 500" 759t, 756t, 760t, 750t,

707* 713" 717" 717"

490*

745 755 759 740,698 701 686 679 730,673* 597

Technique TA TA TA TA TA TA TA TA TA TA TA TA DTA DTA DTA DTA DTA DTA DTA DTA DTA

* Probably undercooled. CThermal effect observed during heating c y c l e - all other TA data are from cooling curves.

330

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Phase relations in the system KF-CeF~

333

Table 3. The more important invariant equilibria in the KF-CeF3 system Type Equilibrium Eutectic Peritectic Peritectic Peritectic Eutectoid

Composition (mole % CeF~)

Temp. (°C)

19 24 32 63 65

620 + 3 675 _+ 10 755 ± 5 1135 --+ 15 650 ± 12

Equilibrium phases present KF. 3 KF'CeF3, liquid KF.CeF~, 3 KF-CeF:~, liquid Cubic ss, KF'CeF3, liquid CeF:. Cubic ss, liquid Cubic ss, KF.CeF~,,SKF.2CeF:~ ss

s s + Liquid" had crystalline solid solution of constant refractive index plus

varying amounts of liquid (quench growth). The decomposition temperature decreases with increasing CeF3 content above 50 mole % to a minimum of 650 ° at 65 mole % CeF3 (eutectoid) and then increases to 695 ° at 66-2/3 mole % (aKF.2CeF3). The 66-2/3 mole % CeF3 composition represents the pure phase aKF.2CeF3 in the temperature range from 695 to 1135°C. Compositions in the region labeled "Cubic ss" that were quenched at 760°C varied in refractive index from 1.486 at 50 mole % CeF3 to about 1.526 at 66-2/3% CeF3. Only the one crystalline phase was observed in all samples quenched at this temperature between these composition limits. The CeF3 primary phase field above 1135°C is partly inferred for compositions containing more that 66-2•3 mole % CeF3 because our experimental temperatures were limited to 1190°C. A small variation in the refractive index of CeF3 which was observed in the quenched samples may indicate a slight solubility of KF in CeF3 at high temperatures but, because of the temperature limitations, we were unable to establish the extent of the solubility. Alpha KF.2CeF3 is replaced by a phase which is designated/3KF.2CeF:~ in Fig. 2. This/3 phase is distinguished by a slight birefringence in those compositions containing more that 66-2•3 mole % CeF3 between the temperature limits of 585°C and 695°C. The X-ray diffraction patterns, as well as the optical properties, of the aKF.2CeF3 and the/3KF-2CeF3 phases are quite similar. It is only in this limited temperature range that/3KF-2CeF3 exists as a stable phase. It has a solid solubility for KF.CeF3 over a very narrow composition range less than 66-2/3 mole % CeF3 that could not be accurately defined. This region is shown in the diagram by dotted lines. The phase fields of cubic solid solution and cubic solid solution +/3KF'2CeF3(ss) were located in spite of the above mentioned small difference in optical properties of the two phases. We had to hold the quench tubes at temperature for 3 weeks to obtain quenched samples that were crystallized well enough to define the equilibrium relations in this part of the diagram. Some of the quenching data in Table 2 that were obtained with shorter equilibration periods are at variance with Fig. 2 for this reason. Our diagram for the KF-CeF3 system differs in several respects from the partial diagram reported by Bukhalova and Babaeva [ 12]. We find a eutectic melting point at 620°C as compared to their value of 710 ° but we agree rather well on the eutectic composition. We show an upper stability limit of 675 ° for the 3KF'CeF3 compound, 47 ° higher than their value and they apparently overlooked the lower

334

C.J. B A R T O N el al.

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Phase relations in the system KF-CeF:~

~35

stability limit of this compound. Their value of 788 ° for the incongruent melting point of KF'CeF3 compares with our figure of 755 °. The cause of the discrepancies is not obvious but our experience has shown that equilibrium data in this system can only be obtained by a technique such as gradient quenching that allows adequate time for equilibrium to be achieved within the temperature range of equilibration followed by rapid cooling to "freeze in" the phases that exist at high temperatures. The statement[15] that 3KF.CeF:~ is cubic is definitely at variance with our observations that this compound is hexagonal in samples quenched to room temperature from the narrow temperature range where it is stable. Our diagram shows that KF and CeF3 should not react to form 3KF-CeF:~ at 500°C, the temperature used by Besse and Capestan [15] for this preparation. It is interesting also to compare the diagram in Fig. 2 with that reported by Schmutz[6] for the KF-NdF3 system. He postulates the same three compound compositions that we found and corresponding equilibria in the two systems occur at very nearly the same temperatures. Schmutz's diagram is based on DTA data and on X-ray examination of ~t comparatively few quenched samples. Use of the gradient quenching technique in ore- investigation permitted examination of many more quenched samples than Schmutz studied and we believe that the KF-NdF:~ system is more complex than is shown in Schmutz's diagram. As was noted in the introduction, Schmutz obtained X-ray diffraction patterns for 3KF •CeF3 and KF-2CeF~ which, according to our studies, are not stable below 595 ° and 585 °, respectively. Our thermal analysis and DTA curves indicate that the decomposition of both compounds occurs rather sluggishly and it would not be surprising if they exist in a metastable state at room temperature. Schmutz noted that the X-ray pattern for the 3 KF'CeF3 preparation was poor. A cknowled,~,ements - W e wish to thank M. A. Bredig and J. E. Ricci t\~r helpflll suggestions.