Synthesis and characterization of mixed fluorides Y1−xCa2+1.5xF7 (−1.33 ≤ x ≤ 1.0)

Materials Research Bulletin, Vol. 34, Nos. 12/13, pp. 2093–2100, 1999 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(99)00218-4

SYNTHESIS AND CHARACTERIZATION OF MIXED FLUORIDES Y1ⴚxCa2ⴙ1.5xF7 (ⴚ1.33 < x < 1.0)

S.N. Achary, S.J. Patwe, and A.K. Tyagi* Applied Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India (Refereed) (Received February 1, 1999; Accepted February 1, 1999)

ABSTRACT In this paper, we report on the synthesis and characterization of the Y1⫺xCa2⫹1.5xF7 (⫺1.33 ⱕ x ⱕ 1.0) compounds and the phase equilibria in the CaF2–YF3 system, in the continuation of our earlier studies on Sr and Ba analogues. It was found that the solubility limit of YF3 in the CaF2 lattice is about 33 mol% as compared to about 25 mol% of YF3 in BaF2 and SrF2. The CaF2–YF3 system shows the formation of an ordered cubic superstructure after the solubility limit. A hexagonal tysonite type phase separates out beyond 41 mol% of YF3. A single phasic tysonite type product was obtained at the composition with about 75 mol% of YF3, i.e., Y1.9Ca0.65F7. © 2000 Elsevier Science Ltd

KEYWORDS: A. fluorides, C. X-ray diffraction, D. phase equilibria

INTRODUCTION Fluorides in general have interesting optical [1,2], magnetic [3,4], and ion-conducting [5] properties. Many studies have been made to prepare new stable fluorides phases. In the context of ionic conductivity, much attention has been paid to the alkaline-earth fluoride systems, where the optimum aliovalent cation substitution leads to an increase in ionic conductivity [6,7]. In the quest of stable single phasic compounds, it is essential to

*To whom all correspondence should be addressed. Fax: ⫹91-22-550-5151 or 22-551-9613. E-mail: [email protected]. 2093

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investigate the phase equilibria in various systems. Earlier we reported [8 –11] the phase equilibria in several rare-earth oxyfluorides and in MF2–YF3 (M ⫽ Sr and Ba) systems with the general composition Y1⫺xM2⫹1.5xF7. In this communication, we report the phase equilibria in the CaF2–YF3 system. The first study on the CaF2–YF3 system was conducted by Vogt [12], who reported the fusibility curve in this system. Subsequently, Goldschmidt et al. [13] suggested a valence compensation by an additional fluorine at the center of empty cubes of the fcc CaF2 lattice, to describe the formation of fluorite-type solid solution. A more detailed study on this system was published by Short and Roy [14]. They proposed a phase diagram of this system and supported the valence compensation model. They reported a very high solubility (about 55– 60 mol%) of YF3 in CaF2, with the fluorite lattice retained. Beyond this solubility limit, they observed a mixture of cubic and hexagonal tysonite-type phases. According to them, the hexagonal tysonite-type phase is a line compound with the composition CaF2– 4YF3. They found that YF3 separates out and coexists with the tysonite-type phase beyond this composition, towards the YF3-rich side. Subsequently, Seiranian et al. [15] published a different phase diagram. These authors found the solubility limit to be 37.4(5) mol% of YF3 in the CaF2 lattice. They proposed the existence of the tysonite-type phase in a range of composition in the high-temperature quenched region. An elaborate study was made by Sobolev and his coworkers [16,17]. They published detailed phase diagrams of the CaF2–(Y,Ln)F3 systems for all the lanthanides except Pm and Eu, in the temperature range of above 800°C. The solubility limit of YF3 in CaF2 lattice was found to be about 38 mol%. The existence of fluorite- and tysonite-type phases was further supported by them. They reported that the tysonite-type phase exists in a well-defined range of compositions. In addition, they mentioned the existence of a phase with an ordered anion sublattice corresponding to the tysonite-type phase, which itself is a disordered phase with respect to the cation sublattice, i.e., having a random distribution of the Ln3⫹ and Ca2⫹ at the La3⫹ sites of LaF3. The authors found [16,17] an increase in the space occupation as a function of the Ln3⫹ content in the fluorite-type phase, leading to an increase in molar volume. Concurrently, Gettmann and Greis reported [18] a number of ordered phases in long-annealed samples in the CaF2–YF3 system, namely, Ca2YF7 (tetragonal), Ca9Y5F33 (rhombohedral, rh␣), Ca8-␦Y5⫹␦F31⫺␦ (rhombohedral, rh␤), and Ca3Y7F27 (monoclinic). The tysonite-type phase was reported to exist with about 70 –92.5 mol% of YF3 in CaF2. Different investigations on the CaF2–YF3 system have reported different stoichiometry of the tysonite-type phase, e.g., CaF2– 4YF3 [14], 5CaF2–13YF3 [19], and 2CaF2–5YF3 [20]. All of the above studies were made on samples either quenched from the melt or equilibrated at high temperatures and, hence, only the high-temperature phases were identified. However, there is no report to date on the stable phases obtained by short heatings and without retaining high-temperature phases, i.e., under slow-cooled conditions. The present study was aimed to establish the compositions of the stable phases obtained after the slow cooling of the short annealed samples and to determine the low-temperature phase equilibria in the CaF2–YF3 system. The results of this study were intended to supplement earlier high-temperature results, to yield the phase equilibria in the complete range of temperatures. In addition, it was considered worthwhile to reinvestigate the composition of the tysonitetype phase in this system.

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EXPERIMENTAL CaF2 was prepared by treating CaCO3 with 40% HF and subsequently drying the product in an inert atmosphere. The YF3 was prepared by a repeated reaction between Y2O3 and NH4HF2 at 400°C and dried in an inert atmosphere. In order to prepare general compositions Y1⫺xCa2⫹1.5xF7 (⫺1.33 ⱕ x ⱕ 1), appropriate amounts of CaF2 and YF3 were ground together to make homogeneous mixtures. About 300 mg of each mixture was pressed into a pellet of 6 mm diameter, wrapped in a platinum foil, and sealed in an evacuated quartz tube. The tube was heated at 900°C for 8 h and cooled back to room temperature at a rate of 2°C/min. XRD patterns were recorded on a Philips PW 1710 X-ray diffractometer using Ni-filtered Cu K␣ radiation; silicon was used as an external standard. The recorded XRD patterns were analyzed by comparing with them with reported patterns.

RESULTS AND DISCUSSION XRD patterns of some of the compositions and details of the phases are given in Figure 1 and Table 1, respectively. The compositions were varied from x ⫽ ⫺1.33 to 1.0, which correspond to the end members YF3 and CaF2. The XRD patterns of the starting materials were found to match those reported [21]. The systematic change in XRD patterns were interpreted for the phases present and further confirmed by indexing the observed reflections. The complete range of compositions in CaF2–YF3 system was identified to contain several phases, e.g., fluorite-type phase, a new kind of cubic ordered phase, hexagonal (tysonite-type) phase, and O–YF3 phase. The first part of this range, from Y0.0Ca3.50F7 to Y1.0Ca2.00F7, contained the fluorite (CaF2)-type phases. In the case of Y1.0Ca2.00F7, we observed a cubic phase along with a very weak hump at 2␪ ⬇ 43.8°, which became prominent and appeared as a proper peak at the composition Y1.1Ca1.85F7 (where about 37.3 mol% of YF3 was present). Hence, we conclude that 33.3 mol% of YF3 is soluble in the CaF2 lattice without any distortion or formation of the ordered superstructure. It should be noted that there is a disagreement between various authors about the solubility limits of YF3 in CaF2 lattice, e.g., 55 [14], 37.4 [15], 38 [16], and 37.5 mol% YF3 [18]. The very high solubility obtained by Short and Roy [14] might be due to an incorporation of oxygen at the fluorine site. In the present investigation, we found that a maximum of 33.3 mol% YF3 could be dissolved in the CaF2 lattice and thereafter it showed an ordering. Gettmann and Greis [18] showed that up to about 50 mol% YF3 could be retained without any change in lattice parameter beyond the saturation limit. This may be due to the saturation of the unit-cell volume expansion. However, since their XRD investigation was performed on quenched samples, a higher solubility of YF3 in the fluorite lattice is not surprising. We observed that the unit-cell volume of these phases gradually increased linearly on going from the parent CaF2 lattice to the saturation limit, i.e., Y1.0Ca2.00F7 (Table 1). On the other hand, we found [10,11] a gradual decrease in lattice parameter in the case of SrF2/BaF2–YF3 systems (Fig. 2). This type of expansion was also observed by Sobolev and Fedorov [16] and Gettmann and Greis [18]. The change in unit-cell volume is a combined effect of the ionic radii and inter-ionic repulsion between the interstitial fluoride ions [22]. The formation of interstitial fluoride ions can be explained on the basis of defect chemistry of the CaF2–YF3 system [23]. In the case of SrF2/BaF2–YF3 systems, the effect of ionic radii predominates

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FIG. 1 Powder XRD patterns of some typical compositions in the Y1⫺xCa2⫹1.5xF7 system:(A) Y0.0Ca3.5F7, (B) Y1.0Ca2.00F7 , (C) Y1.1Ca1.85F7, (D) Y1.2Ca1.70F7, (E) Y1.8Ca0.80F7, (F) Y1.9Ca0.65F7, (G) Y2.0Ca0.50F7, and (H) Y2.33Ca0.00F7.

over inter-ionic repulsion. As the content of YF3 in the CaF2 lattice increased, the intensity of the 200 and 222 reflections of the CaF2, which were suppressed in the fcc fluorite lattice, increased, as shown in Figure 1. Gettmann and Greis [18] could retain some ordered phases, e.g., tetragonal YCa2F7 (33.3 mol% YF3); rhombohedral (rh␣) and rhombohedral (rh␤) phases at about 35.7 and 38.5 mol% YF3, respectively, in long-annealed samples. However, in the present investigation, these phases were not observed. We observed some ordering for the composition

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TABLE 1 Compositions and the Phase Equilibria in the Y1⫺xCa2⫹1.5xF7 System Nominal Sample composition

mol% YF3

x

1 2 3 4 5 6 7 8 9 10 11 12 13

Y0.0Ca3.50F7 Y0.1Ca3.35F7 Y0.2Ca3.20F7 Y0.3Ca3.05F7 Y0.4Ca2.90F7 Y0.5Ca2.75F7 Y0.6Ca2.60F7 Y0.7Ca2.45F7 Y0.8Ca2.30F7 Y0.9Ca2.15F7 Y1.0Ca2.00F7 Y1.1Ca1.85F7 Y1.2Ca1.70F7

0.00 1.0 2.90 0.9 5.88 0.8 8.95 0.7 12.12 0.6 15.38 0.5 18.75 0.4 22.22 0.3 25.81 0.2 29.51 0.1 33.33 0.0 37.29 ⫺0.1 41.38 ⫺0.2

14

Y1.3Ca1.55F7

45.61 ⫺0.3

15

Y1.4Ca1.40F7

50.00 ⫺0.4

16

Y1.5Ca1.25F7

54.55 ⫺0.5

17

Y1.6Ca1.10F7

59.26 ⫺0.6

18

Y1.70Ca0.95F7

64.15 ⫺0.7

19

Y1.8Ca0.80F7

69.23 ⫺0.8

20 21

Y1.9Ca0.65F7 Y2.0Ca0.50F7

74.51 ⫺0.9 80.00 ⫺1.0

22

Y2.1Ca0.35F7

85.71 ⫺1.1

23

Y2.2Ca0.20F7

91.67 ⫺1.2

24

Y2.3Ca0.05F7

97.87 ⫺1.3

25

Y2.33Ca0.00F7 100.0

⫺1.33

Phase(s) present C C C C C C C C C C C Csuper Csuper Ha Csuper H Csuper H Csuper H Csuper H Csuperb H Csuperb H H H O H O H O H O O

a (Å)

b (Å)

c (Å)

V (Å3)

Angle (°)

5.463(2) 5.468(1) 5.474(1) 5.481(1) 5.485(1) 5.489(2) 5.495(1) 5.503(1) 5.509(1) 5.514(2) 5.524(1) 11.071(5) 11.067(1) — 11.070(5) 6.775(1) 11.072(4) 6.773(1) 11.074(4) 6.778(1) 11.075(3) 6.781(1) — 6.776(1) — 6.770(2) 6.781(1) 6.781(1) 6.358(2) 6.780(1) 6.360(3) 6.774(3) 6.360(2) 6.770(5) 6.357(2) 6.362(1)

5.463(2) 5.468(1) 5.474(1) 5.481(1) 5.485(1) 5.489(2) 5.495(1) 5.503(1) 5.509(1) 5.514(2) 5.524(1) 11.071(5) 11.067(1) — 11.070(5) 6.775(1) 11.072(4) 6.773(1) 11.074(4) 6.778(1) 11.075(3) 6.781(1) — 6.776(1) — 6.770(2) 6.781(1) 6.781(1) 6.856(3) 6.780(1) 6.853(5) 6.774(3) 6.859(3) 6.770(5) 6.853(2) 6.854(1)

5.463(2) 5.468(1) 5.474(1) 5.481(1) 5.485(1) 5.489(2) 5.495(1) 5.503(1) 5.509(1) 5.514(2) 5.524(1) 11.071(5) 11.067(1) — 11.070(5) 6.964(1) 11.072(4) 6.964(1) 11.074(4) 6.965(1) 11.075(3) 6.963(2) — 6.964(1) — 6.953(2) 6.969(1) 6.958(1) 4.394(2) 6.960(1) 4.396(3) 6.973(5) 4.391(2) 6.973(6) 4.384(2) 4.391(1)

163.1(1) 163.5(1) 164.0(0) 164.7(1) 165.0(1) 165.4(1) 165.9(1) 166.6(1) 167.2(0) 167.6(1) 168.6(0) 1356.9(1) 1355.6(1) — 1356.7(1) 276.85(1) 1357.4(1) 276.6(1) 1358.2(1) 277.1(1) 1358.5(1) 277.2(1) — 276.9(1) — 276.0(2) 277.5(1) 277.1(1) 191.5(1) 277.1(1) 191.6(1) 277.1(3) 191.5(1) 276.8(4) 191.0(1) 191.5(1)

— — — — — — — — — — — — — — — 120 — 120 — 120 — 120 — 120 — 120 120 120 — 120 — 120 — 120 — —

C ⫽ cubic CaF2 (fluorite) type; Csuper ⫽ cubic superstructure; H ⫽ hexagonal (tysonite) type; O ⫽ orthorhombic YF3 (room temperature modification). a Not refined due to very insignificant intensities of reflections of tysonite phase. b Not refined due to very insignificant intensities of superstructure reflections.

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FIG. 2 Variation of lattice parameters as a function of YF3 content in fluorite structure for Y1⫺xM2⫹1.5xF7 (M ⫽ Ba, Sr, Ca).

Y1.1Ca1.85F7, i.e., with 37.3 mol% YF3 in the CaF2 lattice, which was characterized by a weak hump at the 2␪ ⬇ 34.6° and an additional weak peak at 2␪ ⬇ 43.8°. The XRD pattern of the Y1.1Ca1.85F7, including the peak at 2␪ ⬇ 43.8°, could be indexed on a cubic unit cell with lattice parameter a ⫽ 11.071(5) Å. This type of cubic unit cell was reported [10] by us for Y1.1Sr1.85F7 and Y1.2Sr1.70F7. A similar composition was reported by Sobolev et al. [24] in the SrF2–YF3 system, but with a lattice parameter about twice that obtained by us, i.e., approximately 4 times that of the cubic fluorite cell. Since we did not observe any significant peak at lower angles, the lattice parameter observed by us was about twice that of the fluorite unit cell. The next several compositions, including Y1.2Ca1.70F7, Y1.3Ca1.55F7, Y1.4Ca1.40F7, Y1.5Ca1.35F7, and Y1.6Ca1.20F7, were found to contain another phase in addition to the cubic ordered phase. At the composition Y1.7Ca1.05F7 and Y1.8Ca0.95F7, the intense fluorite reflections were observed, but the weak superstructure reflection at 2␪ ⬇ 43.8° was no longer present. This may be because of the small amount of the superstructure phase. At the composition Y1.9Ca0.65F7, an additional phase, which started appearing from Y1.2Ca1.70F7 onwards, existed as a single phase and no reflections attributable to the fluorite unit cell were observed. This phase could be unequivocally indexed on a hexagonal unit cell with lattice

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parameters a ⫽ 6.781(1) and c ⫽ 6.969(1) Å. These lattice parameters were observed for the hexagonal LaF3 (tysonite); thus, we consider this composition to be the tysonite-type phase that corresponds to 74.5 mol% YF3. On the basis of this, the formula CaF2–3YF3 can be proposed for the tysonite phase. However, other compositions have previously been suggested for the tysonite-type phase: CaF2– 4YF3 [14], Y0.7Ca0.3F2.7 [16], CaF2 with 70 –90 mol% YF3 [18], 5CaF2–13YF3 [19], and 2CaF2–5YF3 [20]. The different heat treatments used by the various groups might be responsible for this disagreement. Further addition of YF3 to the nominal composition Y1.9Ca0.65F7 to give highly Y-rich compositions such as Y2.0Ca0.50F7, Y2.1Ca0.35F7, Y2.2Ca0.20F7, and Y2.3Ca0.05F7 was not successful, because of the separation of O–YF3 (room-temperature orthorhombic phase of YF3) in addition to the tysonite-type phase. Hence, we conclude that a maximum of 75 mol% YF3 can be incorporated into the lattice of CaF2 while yielding a single phasic composition under slow-cooled conditions. It should be mentioned, however, that Gettmann and Greis [18] observed the tysonite-type phase up to a maximum of 90 mol% YF3. We attribute their observation to the quenching of samples. Under our experimental conditions, these phases decompose to give tysonite phase and, accordingly, an additional O–YF3. Further support for the single phasic nature of Y1.9Ca0.65F7 could be inferred from the comparison of lattice parameters in Table 1. We observed that the tysonite-type phase formed in different compositions has approximately the same lattice parameters. Such similarity is observed in O–YF3 and in the cubic superstructure phases. Hence, we suggest that these phases exist only in a narrow range of compositions. It is inferred from the lattice parameters that CaF2 is not at all soluble in O–YF3. CONCLUSIONS These investigations complete a series of studies, establishing the room-temperature phase equilibria in the CaF2–YF3 system for the first time. The Y1⫺xCa2⫹1.5xF7 system shows cubic fluorite-type, ordered cubic, and hexagonal tysonite-type phases. The cubic ordered phase and tysonite-type phase exist in a narrow range of composition, i.e., Y1.1Ca1.85F7 (⬇37 mol% YF3) and Y1.9Ca0.65F7 (⬇75 mol%YF3), respectively. It is concluded that the solubility of YF3 in MF2 lattice increased with a decrease in size difference between M2⫹ and Y3⫹ [10,11]. The BaF2–YF3 system was dominated by the presence of the Y2BaF8 line compound, whereas the tysonite-type phase was dominant in the SrF2/CaF2–YF3 systems. The SrF2–YF3 system was found to have a higher number of ordered phases, compared with the Ba/Ca–YF3 systems. Another striking observation was that no MF2 (M ⫽ Ca, Sr and Ba) is soluble in the YF3 lattice. REFERENCES 1. 2. 3. 4. 5. 6.

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