Interaction of oxyfluorides of rare earth elements with fluorides having the fluorite structure

Journal of the Less-Common Metals, 76 (1980) 55 - 62 @ Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands

55

INTERACTION OF OXYFLUORIDES OF RARE EARTH ELEMENTS WITH FLUORIDES HAVING THE FLUORITE STRUCTURE*

V. A. GORBULEV,

P. P. FEDOROV

Shubnikov (U.S.S.R.)

of Crystallography,

Institute

and B. P. SOBOLEV U.S.S.R.

Academy

of Sciences,

Moscow

(Received September 2,198O)

Summary Wide regions of solid solutions based on metal fluorides and high temperature modifications of rare earth oxyfluorides (ROF) were observed. Single crystals of solid solutions of calcium, strontium and barium fluorides with up to 10 mol.%‘ROF were prepared by the Bridgman technique.

1. Introduction Fluorides and oxides of rare earth elements (R) possess many interesting properties and are used (in particular, in the form of single crystals) as host materials for lasers, as converters of IR radiation into visible radiation, as solid electrolytes etc. Powdered oxyfluorides of rare earth elements of stoichiometric composition ROF find application as matrices for luminophores [l] ; they possess a sufficiently high ionic conductivity [2] . More detailed studies and applications of rare earth compounds containing oxygen and fluorine are hindered by difficulties in preparing single crystals. This is due to the fact that high temperature cubic modifications of oxyfluorides having the fluorite structure and variable composition where x > 0 (we have confirmed this using high temperature ROl-,Fi+z,, X-ray diffraction analysis). decompose with decreasing temperature to form ordered phases: rhombohedral ROF, tetragonal and orthorhombic [3 - lo]. Small (several cubic millimetres in size) single crystals of rare earth oxyfluorides (R = Gd, Dy, Er, Tb, Ho) have been prepared [ 111 by flux crystallization. The synthesis of single crystals of LaOI _x Fi + sx by the Bridgman technique has been described previously [ 121. These crystals were anisotropic and in most cases had a composition that was different from stoichiometric ROF. Literature data on the formation of solid solutions of ROF with fluorides of alkaline earth metals (M) having the fluorite structure are given *Dedicated to Professor Wilhelm Klemm on the occasion of his 85th birthday.

56

in ref. 13. The following scheme of heterovalent isomorphic substitution been proposed for such solid solutions (see, for example, ref. 14): M2++F-

)

R3+ + G2-

has (1)

The scheme (Feofilov’s model) is based on spectroscopic [ 15, 161 and electron spin resonance data [ 171 for MF,-based solid solutions with an ROF content of up to 0.2 wt.%. The object of our work was to study the phase diagrams of MF,-ROF systems (M = Ca, Sr, Ba; R = La, Gd, Y) in order to determine regions of isomorphic substitution and to prepare from the melt single crystals with a mixed anion sublattice. 2. Experimental

procedures

The oxyfluorides ROF were synthesized from RFs and RsO3 (of not less than 99.98%) by sintering at 1100 - 1250 “C for 3 - 9 h in a graphite crucible in an ultrapure helium atmosphere. Low temperature rhombohedral modifications with the following lattice parameters were obtained: for LaOF aa = 7.132(5) A, aR = 33.01(5)“; for GdOF aa = 6.800(5) A, (YR = 33.05( 5)” ; for YOF aR = 6.666(5) A, (Ya = 33.09(5)“. These results coincide with literature data within the limits of error [4, 6, 18, 191. The temperatures of polymorphic transitions determined by differential thermal analysis (DTA) for LaOF, GdOF and YOF are 487, 597 and 569 “C, and agree well with data given in refs. 5 and 20. Quantitative analysis of fluorine confirmed that the compositions of the synthesized samples are close to stoichiometric and can be described by the formula ROlfXFIT 2x, where 3c is less than 0.03. The MF,-ROF systems were studied using DTA and X-ray diffraction analysis of annealed and quenched samples. The DTA was carried out in an installation with a graphite resistance heater in a helium atmosphere in molybdenum crucibles within the temperature range 300 - 1600 “C. The temperature was measured with W/5%ReW/20%Re thermocouples calibrated via the melting points of CaF, (1418 “C) [ 211, SrF, (1464 “C) [ 22, 231, BaF, (1354 “C) [ 23, 241 and LiF (845 “C) [ 23, 241. The temperature determined was accurate to within +lO “C. The samples were presintered in a DTA installation at 1200 “C for 8 - 10 h. Solidus, liquidus and peritectic temperatures were registered by the beginning of thermal effects on heating and cooling curves respectively (10 20 “C min-‘) ; the temperatures of polymorphic transitions were recorded from heating curves only (10 “C min-’ ) because of the existence of thermal hysteresis. Each sample was analysed at-least twice. The samples for X-ray diffraction analysis were annealed in graphite crucibles in a helium atmosphere and were quenched at a rate of about 5 “C s-l. At 1050 - 1440 “C!the annealing duration was 23 - 10 h. At 1520 1750 “C the samples, sintered previously at 1200 “C for 12 h, were annealed for 60 - 15 min. Constant composition was checked by weighing the samples. purity

Weight losses determined by evaporating the starting components for 1 h under identical conditions were never more than 0.02 wt.% at 1250 “C and 0.04 wt.% at 1410 “C; for BaF, the losses were 0.25 and 0.4 wt.% respectively. Annealing was also performed in an argon atmosphere in a sealed nickel container at 1200 “C for 190 h, followed by quenching at a rate of about 200 “C s-l. Both methods of annealing yielded the same results. To specify the solidus curves for solid solutions based on ROF, the beginning of melting during the sintering of a sample at a given temperature was observed. The accuracy of determining the phase boundaries in the solid solution range was * 5 mol.%. 3. Results and discussion 3.1. Phase diagrams The phase diagrams we obtained are presented in Fig. 1. No continuous solid solutions were observed in any of the systems. The coordinates of invariant points are given in Table 1. The (Ca,Sr)F,-ROF and BaFs-LaOF systems are of a simple peritectic type; the BaF,-YOF system is of a eutectic type. In the BaF,-GdOF system the solid solution based on BaF, reacts peritectically but has a minimum on the melting curve at 1.5 mol.% GdOF and 1345 “C (see Fig. 1). At high temperatures extended regions of the componentrbased solid solutions are observed in all the systems; however, with decreasing temperature the homogeneity regions are sharply reduced. The temperatures of the ROF phase transitions are only slightly reduced in binary systems (by 12 “C for YOF in the system with CaF, and by about 5 “C for LaOF with CaF,) which means that the high temperature modifications are almost not stabilized. Solid solutions based on fluorides of the alkaline earth metals also decompose rapidly. The concentration dependences of the elementary cell parameters in some MF,-ROF systems are given in Figs. 2 - 4. It is noteworthy that the curves show strong positive deviations from linearity (Vegard’s law); these deviations are up to 0.045 A in the CaFs-LaOF system (Fig. 2). The ROF distribution coefficients k near MFs determined from the DTA data in the systems of the peritectic type were found to vary from 4 f 2.5 to 10 f 2.5 (the accuracy of measuring the distribution coefficients was limited by that of measuring the temperature). The exceptions (k < 1) are the BaFs-GdOF (peritectic with a minimum) and BaFs-YOF (eutectic) systems. The fact that the distribution coefficients differ markedly from unity means that it is very difficult to prepare MF, crystals doped with ROF with a uniform distribution of ROF. From this standpoint the minimum of the liquidus curve determined by us in the BaFs-GdOF system is of great advantage (Fig. 1). The anomalous shape of the solubility curve in the solid state in the SrF,-GdOF, SrF,-YOF and BaF2-LaOF systems also attracts attention. In these systems the solubility increases sharply when the temperature exceeds values close to the peritectic line (Fig. 1).

T°C

CaF2

I

I

I

I

SrF,

20

40 60 mote x ROF

Fig. 1. MFz-ROF phase diagrams: specimens or partial melting.

l

80

Bi%F2

, DTA data; 0, single-phase

-.

specimens;

@, two-phase

Analysis of experimentally obtained phase diagrams of MFs-ROF systems (Fig. 1 and Table 1) reveals certain deviations from the regularities usually observed in systems with soluble isostructural components in the solid state. In particular in MFs-ROF systems maximum solubility does not correspond to maximum closeness of the lattice parameters of the components (see Tables 2 and 3). As we shall discuss in a forthcoming publication [29], this phenomenon, as well as the formation of wide regions of solid solutions in the systems studied, cannot be explained using the assumption that the ions are randomly distributed in the crystal lattice. The systems studied in this work are similar to the peritectic quasibinary section N%.rY0.6F2.2- YOF [30]. The Nae4YoVsF2, phase has a fluorite-type structure with a lattice parameter a of 5.508 A and a congruent melting point of 984 “C.

1

coordinates.

1385 1353 1323

BaF2-LaOF BaFz-GdOF BaF2-YOF

aEutectic

1500 1490 1480

SrFz-LaOF SrF,-GdOF SrFz-YOF

* 10 f 10 k 1Oa

f 10 f 10 + 10

f 10 + 10 * 10

1454 1457 1454

CaFz-LaOF CaF2-GdOF CaF,-YOF

points

Peritectic temperature W)

of invariant

System

Coordinates

TABLE

67+ 172 2Ok

39k 45* 40*

45* 4Ok 25t

5 5 5

5 5 5

5 5 5 5 5 5

18k 5 17+ 5 23 + 5

57* 15* 30*

45* 5 30+ 5 27 + 5

MF,)

(mol.%

(mol.%

ROF)

Maximum homogeneity region of solid solution based on ROF

systems

Maximum homogeneity region of solid solution based on MF2

in MF,--ROF

2 2 2

12? lo+ 32.5

ROF)

2 2 f 2a

6+2 12+ 2 9+ 2

lo? 7? 5i-

(mol.%

Liquid composition at the peritectic temperature --

487 597 569

470 597 569

482 597 557

T CC)

Eutectoid

= 100 = 100 = 100

97 = 100 2 100

96 =lOO 96

(mol.%

ROF)

decomposition

60 a.i

Cd F,

20

40

60

Fig. 2. Lattice constants in the CaFz-LaOF 1400 ‘C; 0, at 1245 “C; +, at 1060 “C.

80

uo".,jdLaw

system us. the LaOF concentration:

0, at

a.d

545 cd&

20

4a

60

Fig. 3. Lattice constants in the CaFz-GdOF 1400 ‘C; +, at 1200 “C.

Fig. 4. Lattice constants in the CaFz-YOF 1400 “C; +, at 1230 “C.

80

Aml,%&Of

system us. the GdOF concentration:

system vs. the YOF concentration:

0, at

0, at

61 TABLE 2 Parameters of elementary cells of compounds having the fluorite structure Compound

a (A)

Reference

Compound (high temperature modification)

a (4

Reference

CaF2

5.463

25

LaOF

5.756 5.767 5.73

3 26 This work (extrapolation)

SrFz

5.800

25

GdOF

5.5U6a 5.515

27 This work (extrapolation)

BaF2

6.200

25

YOF

5.363 5.39

28 This work (extrapolation)

aNon-stoichiometric

composition.

TABLE 3 Difference Aa/a,i, (%) between elementary cell parameters of the constituents of binary MF,-ROF systems

CaF, SrF, BaF,

YOF

GdOF

LaOF

1.9 8.2 15.7

0.8 5.3 12.5

5.7 0.5 7.5

growth Single crystals of MFs-based solid solutions were grown by the Bridgman technique in graphite crucibles in a helium atmosphere at rates of 10 and 25 mm h-l. A slow cooling of the crystals down to room temperature was accomp~ied by turbidity and the precipitation of a second phase even at 1 mol.% R.OF. This can be explained in terms of decomposition of the solid solutions (Fig. 1). Therefore, we quenched the crystal from temperatures within the solid solution range (1000 - 1200 “C). In this way we prepared optically transparent colourless single crystals of solid solutions in all nine systems studied with a maximum condensation of ROF in the range from 2 to 10 mol.%. 3.2.

Crystal

4. Conclusions (1) The phase diagrams of nine MF,--ROF systems were studied. Wide regions of solid solutions based on MFs and high temperature modifications of ROF were observed; these regions narrowed sharply with decreasing temperature.

62

(2) Single crystals of solid solutions based on calcium, strontium and barium fluorides containing up to 10 mol.%‘ROF were grown by the Bridgman technique with subsequent quenching of the crystal.

Acknowledgment The quanti~tive analysis of fluorine was performed by 1;. P, Reshetnikova of the Chemistry Department of Moscow State University.

References 1 V. A. Petrov, F. F. Gavrifov, V. G. Bamburov and B. V. Shul’gin, Spe~tras~op~~ kristallou, Nauka, Moscow, 1975, p. 303. 2 A. Pelloux, P. Fabry and C. Deportes, C. R. Acad. Sci., Ser. C, 276 (1973) 241. 3 W. Klemm and H. A. Klein, 2. Anorg. Allg. Chem., 248 (1941) 167. 4 W. Zachariasen, Acta Crystallogr., 4 (1953) 231. 5 D. B. Shinn and H. A. Eick, Znorg. Chem., 8 (1969) 232. 6 K. Niihara and S. Yajima, Bull. Chem. Sot. Jpn., 44 (1971) 643. 7 D. J. M. Bevan, R. S. Cameron, A. W. Mann, G. Brauer and V. Roether, Znorg. Nucl. Chem. Lett., 4 (1968) 241. 8 A. W. Mann and D, J. M. Bevan, J. Solid State Chem., 5 (1972) 410. 9 A. de Kozak, M. Samouel and A. Chretien, Rev. Chim. Miner., 10 (1973) 259. 10 B. Tanguy, M. Vlasse and J. Portier, Rev. Chim. Miner., IO (1973) 63. 11 G. Garton and B. M. Wanklyn, J. Muter. Sci., 3 (1968) 395. 12 V. V. Osiko, A. A. Sobol’, M. I. Timoshechkin and M. M. Fyrsikov, Tr. Fiz. Inst. Akad. Nciuk SSSR, 60 (1972) 72. 13 S. S. Batsanov, L. I. Kobets and L, I. Kushkova, Kristallographiya, 13 (1968) 422. 14 V. S. Urusov, Theoria izomorphnoj smesimosty, Nauka, Moscow, 1977. 15 P. P. Feofilov, Dokl. Akod. Nauk SSSR, 99 (1954) 731. 16 A. A. Kapljansky, Zzv. Akad. Nauk SSSR, Ser. Fiz., 25 (1961) 20. 17 J. Sierro, J. Chem. Phys., 34 (1961) 2183. 18 N. C. Baenziger, G. R. Holden, G. E. Knudson and A. J. Popov, J. Am. Chem. Sot., 76 (1954) 4734. 19 A. W. Mann and D. J. Bevan, Acta Crystallogr., Sect. B, 26 (1970) 2129. 20 K. Niihara and S. Yajima, Bull. Chem. Sot. Jpn., 45 (1972) 20. 21 B. P. Sobolev, P. P. Fedorov, D. B. Shteynberg, B. V. Sinitsyn and G. S. Shakhkalamian, J. Solid State Chem., 17 (1976) 191. 22 F. Delbove and S. Lailemand-Chatain, C. R. Acad. Sci., Ser. C, 270 (1970) 964. 23 B. Porter and E. A. Brown, J. Am. Chem. Sot., 45 (1962) 49. 24 H. Kojima, S. G. Whiteway and C. R. Masson, Can. J. Ckem., 46 (1968) 2968. 25 ASTM X-ray data cards, ASTM, Philadelphia, Pennsylvania, 1973. 26 L. R. Batsanova and G. N, Kustova, Zh. Neorg. Khim., 9 (1964) 330. 27 V. R. Falikman and F. M. Spiridonov, Vestn. Mosk. Gos. Uniu., (3) (1976) 346. 28 F. Hund, 2. Anorg. Allg. Chem., 265 (1951) 62. 29 V. A. Gorbulev, P. P. Fedorov and B. P. Sobolev, to be published. 30 P. P. Fedorov, B. P. Sobolev and S. F. Belov, Zzu. Akad. Nauk SSSR, Neorg. Mater., 15 (1979) 816.