CHEMISTRY

Yu. M. KORENEV, Corresponding Member of the Academy of Sciences of the USSR, A. V. NOVOSELOVA

STUDY OF THE SYSTEM BeF₂—ZrF₄

In connection with the study of the ternary system KF—BeF₂—ZrF₄, we undertook a study of the corresponding binary systems. The present paper reports the results of an investigation of the BeF₂—ZrF₄ system. We found no data in the literature on this system. According to literature data, the isomorphous zirconium fluorides ThF₄ and UF₄ (¹) and (²) form systems of the eutectic type with beryllium fluoride. No compounds have been found in them. The eutectic points are located very close to beryllium fluoride: at 2 mole % ThF₄ and 0.5 mole % UF₄, which indicates the comparatively low solubility of beryllium fluoride in melts of these fluorides. The eutectic temperatures are, respectively, 525–530° and 537°.

There are also a number of papers in the literature (³–⁵) which indicate that zirconium fluoride forms binary compounds of various composition with fluorides of divalent metals (Mg, Ba, Zn, Cd, Ni). All the compounds indicated were obtained only from aqueous solutions and were isolated in the form of crystalline hydrates.

Fig. 1. Phase diagram of the BeF₂—ZrF₄ system

Thermal analysis of the BeF₂—ZrF₄ system was carried out by us on a Kurnakov FPK-59 pyrometer. The alloy under investigation was placed in a platinum crucible with a tightly fitted lid, which had a pocket for the thermocouple. To avoid loss of zirconium fluoride as a result of evaporation, the alloys were introduced into a furnace preheated somewhat above the melting temperature of the alloy. Analyses for the total oxides of zirconium and beryllium showed no significant deviations from the initial compositions.

For alloys with a content from 0 to 80 mole % BeF₂, cooling curves were recorded. Alloys with a higher content of beryllium fluoride

were obtained in the form of glass; therefore, only the heating curves after annealing the alloys were recorded for them.

For preparing the alloys, mixtures of ((\mathrm{NH}_4)_2\mathrm{BeF}_4) and ((\mathrm{NH}_4)_3\mathrm{ZrF}_7) were taken in calculated amounts, and ammonium fluoride was driven off in a stream of (\mathrm{CO}_2). To avoid melting of the double fluorides during distillation, most of the ammonium fluoride was driven off at (300^\circ); then the temperature was gradually raised to (500\text{--}550^\circ), and the final removal of (\mathrm{NH}_4\mathrm{F}) from the alloys was carried out. Annealing of the alloys was performed in nickel crucibles sealed in ampoules, at a residual argon pressure of 100–120 mm Hg. Argon was used to suppress sublimation of zirconium fluoride. The annealing temperature was (380\text{--}400^\circ), and the time was 30 h.

X-ray studies of the alloys were carried out with Fe radiation in RKD-57 cameras. Alloys with a high (\mathrm{BeF}_2) content were photographed in capillaries made of Pyrex glass; nonhygroscopic alloys were photographed on glass whiskers. The film loading was asymmetric. The data from thermal and X-ray phase analysis of the (\mathrm{BeF}_2)—(\mathrm{ZrF}_4) system are presented in Figs. 1 and 2.

Fig. 2. Comparison of reflection angles and line intensities in X-ray patterns of alloys of the (\mathrm{BeF}_2)—(\mathrm{ZrF}_4) system. (a)—annealing at (400^\circ), (b)—annealing at (550^\circ).

On the heating curves of zirconium fluoride obtained by decomposition of ((\mathrm{NH}_4)_3\mathrm{ZrF}_7) at (550^\circ), endothermic effects were found at 612, 690, and (903^\circ). The effect at (612^\circ) is absent from the thermograms of zirconium fluoride obtained by decomposition of ((\mathrm{NH}_4)_3\mathrm{ZrF}_7) at temperatures above (600^\circ). This effect cannot be attributed to a phase transformation of (\mathrm{ZrF}_4), since the X-ray patterns of (\mathrm{ZrF}_4) obtained at 550 and (650^\circ) proved to be identical.

Nor can it be assumed that the last traces of ammonium fluoride are removed at this temperature, since according to the data of work ((^6)) this occurs at (410^\circ). Probably, at (612^\circ) a second-order transformation of (\mathrm{ZrF}_4) occurs.

The effect at (690^\circ) is reversible and corresponds, as already reported in work ((^7)), to the (\alpha \rightleftarrows \beta) transformation of (\mathrm{ZrF}_4). At (903^\circ) zirconium fluoride melts. Single crystals of (\mathrm{ZrF}_4) that had grown under the lid of a platinum crucible were photographed. The photographs were taken in an RKOP camera with Cu radiation. The study showed that the crystals belong to the monoclinic modification described by Schulz ((^8)).

Beryllium fluoride was obtained by the method described in work ((^9)). It was obtained in a quartz-like form. In this same form it was also found in alloys annealed at (400^\circ).

On the heating curve of (\mathrm{BeF}_2), effects were found at 220, 575, and (793^\circ). The effect at (220^\circ) corresponds to the (\alpha \rightleftarrows \beta) transformation of quartz-like (\mathrm{BeF}_2); (575^\circ) corresponds to melting of this modification. In work ((^{10})) it was found that after melting at (580^\circ), (\mathrm{BeF}_2) solidifies again, passing into a high-temperature modification, which melts at about (800^\circ). Endothermic-

effect at 793° probably also corresponds to the final melting of beryllium fluoride.

As already noted in (11), in melts of $\mathrm{BeF}_2$ and $\mathrm{ZrF}_4$ a new modification of $\mathrm{BeF}_2$ was found, which we indexed in a rhombic body-centered cell with the parameters: $a = 10.01 \pm 0.01$ kX, $b = 13.07 \pm 0.02$ kX, $c = 16.27 \pm 0.01$ kX.

Special experiments showed that the rhombic modification of $\mathrm{BeF}_2$ is formed in the presence of very small amounts of zirconium fluoride and that the crucible material (the experiments were carried out in nickel and platinum crucibles) has no effect on its formation.

To determine the stability region of the rhombic modification of $\mathrm{BeF}_2$, we annealed it, first at 550° for 150 hours, and then at 380–400°. After annealing at 380–400° this modification transformed into the quartz-like form of $\mathrm{BeF}_2$.

In the study of the $\mathrm{NaF}$—$\mathrm{BeF}_2$ system (12), a transition at 420–450° of an then unknown modification of $\mathrm{BeF}_2$ into the quartz-like form was found. Therefore we may regard 420° as the lower boundary of existence of the rhombic modification. The upper boundary, corresponding to the transition of $\mathrm{BeF}_2$ into a more high-temperature form, probably analogous to cristobalite, according to the data of (12) lies at 680°. We marked the upper boundary of existence of rhombic $\mathrm{BeF}_2$ on the diagram by a dashed line (Fig. 1).

By thermal and X-ray phase analysis in the $\mathrm{BeF}_2$—$\mathrm{ZrF}_4$ system, one incongruently melting compound of composition $\mathrm{BeZrF}_6$ was found. The temperature of the peritectic reaction of its formation lies at 712°. The eutectic between this compound and beryllium fluoride lies at a temperature of 578° and 80 mol. % $\mathrm{BeF}_2$.

Moscow State University
named after M. V. Lomonosov

Received
17 I 1963

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