CHEMISTRY

R. G. Grebenshchikov

INVESTIGATION OF THE PHASE DIAGRAM OF THE SYSTEM RbF—BeF₂ AND ITS RELATION TO THE SYSTEM BaO—SiO₂

(Presented by Academician N. V. Belov on 16 XI 1956)

Among fluoride systems of the type Me⁺F—BeF₂, where Me⁺ is an alkali element, phase diagrams have been constructed only for two systems: LiF—BeF₂ (¹,²) and NaF—BeF₂ (³–⁵), which, in the light of Goldschmidt’s concepts, may be regarded as weakened fluoride “models” of the silicate systems Mg(Zn)O—SiO₂ and CaO—SiO₂, respectively.

In order to fill the gap—the absence of a fluoride “model” of the BaO—SiO₂ system—it was of interest to investigate the previously unstudied RbF—BeF₂ system with respect to its chemical compounds and phase diagram.

To construct the phase diagram of the RbF—BeF₂ system, differential-thermal, X-ray phase, and crystal-optical methods of investigation were used. Twenty-five different compositions of the system were subjected to thermal investigation; after each experiment they were examined under a microscope in polarized light in immersion preparations in order to determine the phase composition of the samples from the refractive indices of the individual substances. Some mixtures (15; 40; 46.5; 60; 75 mol.% BeF₂ and several additional compositions) were investigated radiographically for additional control of their phase composition. The RbF—BeF₂ system, shown in Fig. 1, consists of four separate eutectic systems; in the thermal investigation of these, the end members of these partial systems served as the starting substances. The exception is the RbBe₂F₅—BeF₂ system, where BeF₂ was introduced into the starting mixtures in the form of the salt (NH₄)₂BeF₄*, which decomposed in the range 220–380° into BeF₂ and gaseous NH₄F.

The starting substances used in obtaining rubidium fluoroberyllates were Rb₂CO₃ (chemically pure) and (NH₄)₂BeF₄. Three compounds were found in the system: Rb₂BeF₄, RbBeF₃, and RbBe₂F₅. Rb₂BeF₄ was first obtained by Makerji (⁶); the existence of RbBeF₃ is indicated by Novoselova and Simanov (⁷); no data on RbBe₂F₅ are available in the literature. We synthesized Rb₂BeF₄ from an aqueous solution of equimolecular amounts of Rb₂CO₃ and (NH₄)₂BeF₄; RbBeF₃ and RbBe₂F₅ were prepared by sintering Rb₂BeF₄ and RbBeF₃—each separately—with (NH₄)₂BeF₄ in a molar ratio of 1 : 1 in both cases. RbF was obtained from Rb₂CO₃ by displacement of CO₂ with hydrofluoric acid. Questions concerning the study of the polymorphism, structure, and crystal-optical properties of Rb₂BeF₄ and RbBeF₃ as fluoroberyllate “models” of Ba₂SiO₄ and BaSiO₃, respectively, are considered in detail in our works (⁸,⁹). The individuality of rubidium ortho-, meta-, and difluoroberyllates was confirmed by us through measurement of their crystal-optical properties and by data on the interplanar spacings of the lattices of these compounds. Table 1 gives the values of the refractive indices ((N_{\mathrm{av}})), densities, and chemical analysis of (NH₄)₂BeF₄ and of three rubidium fluoroberyllates.

* In view of the low values of the refractive indices of the fluoroberyllates, immersion liquids were prepared from methyl alcohol ((N = 1.329)) and glycerin.

** The salt (NH₄)₂BeF₄ was prepared by the method of Novoselova and Averkova (¹⁰) from equimolecular amounts of Be(OH)₂·(NH₄)₂CO₃ and hydrofluoric acid.

The study of the partial eutectic system RbF—Rb₂BeF₄ presented difficulties because of the hygroscopicity and some volatility of RbF; the latter was detected by chemical analysis of a mixture of eutectic composition after thermal analysis. This apparently also explains the small scatter of points in this part of the system. The eutectic of the RbF—Rb₂BeF₄ system, formed by congruently melting RbF (m.p. 770°) and Rb₂BeF₄ (m.p. 807°),

Table 1

Content of elements in compounds (wt. %)

Fig. 1. Phase diagram of the RbF—BeF₂ system: 1 — temperatures of thermal effects on heating thermograms, 2 — temperatures of thermal effects on cooling thermograms

lies at 693° and has the composition 23 mol.% BeF₂ + 77 mol.% RbF. Solid-phase equilibria in the RbF—Rb₂BeF₄ system are characterized by the existence of 5 stability fields of limited solid solutions of RbF in

Rb₂BeF₄, of which there are: 1) two fields of phase stability: γ-solid solution + β-solid solution (field 6—7—8) and β-solid solution + α-solid solution (field 3—4—5), and 2) three fields of stability of γ-solid solution (field 9—6—8), β-solid solution (field 4—5—7—8), and α-solid solution (field 1—2—3—4). The necessity for the existence of the listed fields follows as a consequence of the three modifications present: γ-, β-, and α-Rb₂BeF₄, which undergo enantiotropic polymorphic transformations according to the scheme

[
\gamma \underset{}{\overset{528^\circ}{\rightleftarrows}} \beta
\underset{}{\overset{692^\circ}{\rightleftarrows}} \alpha
\underset{}{\overset{807^\circ}{\rightleftarrows}} \text{liq.}
]

and each of which forms limited solid solutions with RbF. The exact boundaries of existence of these equilibrium fields have not been established; therefore they are shown by dotted lines. The presence of limited solid solutions of RbF in the three modifications of Rb₂BeF₄ is confirmed by the observed difference in the temperatures of the polymorphic transformations of pure Rb₂BeF₄ in comparison with its limiting solid solutions: for γ-solid solution ⇄ β-solid solution, 556°, and for β-solid solution ⇄ α-solid solution, 672°. In the powder pattern of the composition containing 15 mol.% BeF₂, along with lines belonging to RbF and γ-Rb₂BeF₄ solid solution, there are also sufficiently distinct lines which, in all probability, should be assigned to the β-solid-solution and α-Rb₂BeF₄ solid-solution modifications stabilized by the dissolution of RbF in them. The values (N_{\mathrm{cp}}) of the different modifications of the Rb₂BeF₄ solid solution differed little from the refractive index of γ-Rb₂BeF₄, (N_{\mathrm{cp}}=1.383); therefore the solubility of RbF in γ-, β-, and α-Rb₂BeF₄ was provisionally taken as not more than 10–12 wt.% RbF (at high temperatures).

The portion of the system from 33.33 to 50 mol.% BeF₂ is a eutectic phase diagram Rb₂BeF₄—RbBeF₃, whose eutectic has the composition 47.5 mol.% BeF₂ + 52.5 mol.% RbF and lies at 462°. RbBeF₃ melts congruently at 465°. In the field of separation of Rb₂BeF₄ from the melt, two dotted horizontal lines represent boundaries separating the equilibrium fields of the α and β modifications at 692°, and of β and γ at 528°. Three points corresponding to thermal effects for compositions of 37 and 40 mol.% BeF₂ at an average temperature of 632° correspond, as indicated earlier (*), to the transformation of β-Rb₂BeF₄ into the α′ form, stable in the interval 632–692° in the presence of RbBeF₃. The absence in the RbF—Rb₂BeF₄ system of a similar transformation does not allow it to be represented in the form of corresponding equilibrium fields of the α′ form in the phase diagram of the system. In the system, the existence is assumed of a compound of composition 3RbF·2BeF₂, decomposing in the solid state at 427° into γ-Rb₂BeF₄ and α-RbBeF₃. It was not possible to confirm crystallographically by X-rays the individuality of the compound 3RbF·2BeF₂; microscopically, in mixtures of the Rb₂BeF₄—RbBeF₃ system, the presence was detected of a small amount of a phase with a refractive-index value intermediate between (N_{\mathrm{cp}}) for Rb₂BeF₄ and RbBeF₃. Compounds decomposing in the solid state are, in most cases, unstable; this, apparently, explains the difficulty of obtaining 3RbF·2BeF₂ in an amount sufficient for X-ray confirmation of its chemical individuality. In one of the experiments, the thermal effect for the composition 3RbF·2BeF₂ at 322° was caused by the β ⇄ α transformation of the RbBeF₃ present in it. However, the reason remains unclear for the appearance of a thermal effect in RbBe₂F₅ at a temperature almost coinciding with the temperature of the polymorphic transformation of RbBeF₃. A high-temperature X-ray study of RbBe₂F₅ at 20, 200, and 350° did not reveal any appreciable change in its structure, apart from thermal expansion of the lattice. The horizontal line in the RbF—BeF₂ system for compositions from 40 to 66.66 mol.% BeF₂ at 316° represents the boundary of the fields of existence of the enantiotropic modifications of RbBeF₃, below which rhombic β-RbBeF₃ is stable, and above which α-RbBeF₃ is stable.

The RbF—BeF₂ system in the composition range from 50 to 100 mol.% BeF₂ consists of two simplest eutectic diagrams, RbBeF₃—RbBe₂F₅ and RbBe₂F₅—BeF₂, mixtures of whose compounds crystallize in eutectics of compositions, respectively, 58 mol.% BeF₂ + 42 mol.% RbF at 396° and 77.5 mol.% BeF₂ + 22.5 mol.% RbF at 425°. The crystals of RbBe₂F₅ are biaxial,

with a large angle of the optical axes, a negative (—) optical sign, and very weak birefringence. In crystals of $\mathrm{RbBe_2F_5}$, perfect cleavage along one of the planes and a tendency toward polysynthetic twinning were found. $\mathrm{RbBe_2F_5}$ is slightly hygroscopic, melts congruently at $462^\circ$, and has a flat melting maximum. $\mathrm{RbBe_2F_5}$ forms limited solid solutions with $\mathrm{BeF_2}$—this was established from changes in the interplanar spacings of the solid solution $\mathrm{RbBe_2F_5}$ from a mixture of composition 70 mol.% $\mathrm{BeF_2}$ in comparison with $d/n$ of pure $\mathrm{RbBe_2F_5}$, and also from the decrease, in $\mathrm{RbBe_2F_5}$ solid solution, of the refractive index, which became lower than $N = 1.329$. The presence, in a mixture of composition 70 mol.% $\mathrm{BeF_2}$, of an insignificant amount of a second phase, $\mathrm{BeF_2}$, shows that the composition corresponding to the limiting solubility of $\mathrm{BeF_2}$ in $\mathrm{RbBe_2F_5}$ must lie between $\mathrm{RbBe_2F_5}$ and the composition 70 mol.% $\mathrm{BeF_2}$, as is also shown by the dashed line in the diagram. In the field of the partial diagram $\mathrm{RbBe_2F_5}$—$\mathrm{BeF_2}$, below the eutectic temperature, in equilibrium with $\mathrm{RbBe_2F_5}$ solid solution there is a quartz-like modification of $\mathrm{BeF_2}$, possessing extremely low birefringence.

$\mathrm{BeF_2}$ gradually softens on heating, remaining rather viscous; in addition, it is prone to supercooling. Therefore determining the character and temperature of melting of $\mathrm{BeF_2}$ presents great experimental difficulties. Kirkina, Novoselova, and Simanov ($^{11}$) note that $\mathrm{BeF_2}$ begins to melt at $545^\circ$ and finally melts at about $740^\circ$, but the authors do not indicate the character of its melting. In constructing the phase diagram of the $\mathrm{RbF}$—$\mathrm{BeF_2}$ system, in view of the uncertainty of the melting of $\mathrm{BeF_2}$, we do not bring the liquidus of its separation from the melt up to the temperature ordinate characterizing the thermal behavior of $\mathrm{BeF_2}$.

Comparison of the “model” systems $\mathrm{RbF}$—$\mathrm{BeF_2}$ and $\mathrm{BaO}$—$\mathrm{SiO_2}$ and their phase diagrams shows that compounds similar in stoichiometry of composition, in character of melting (congruent), and in structure are realized in the systems. The phase diagrams of both systems have similar liquidus curves in the composition interval from $\mathrm{RbF}$ to $\mathrm{RbBeF_3}$ in the fluoride system and from $\mathrm{BaO}$ to $\mathrm{BaSiO_3}$ in the silicate system. However, the solid-phase equilibria in the $\mathrm{RbF}$—$\mathrm{BeF_2}$ system are more complex than in $\mathrm{BaO}$—$\mathrm{SiO_2}$. The presence of polymorphism in $\mathrm{Rb_2BeF_4}$ and $\mathrm{RbBeF_3}$, i.e., in the fluoride “models” of $\mathrm{Ba_2SiO_4}$ and $\mathrm{BaSiO_3}$, may serve as a basis for searches for polymorphism also in barium silicates, the existence of which could have remained unnoticed because of the difficulty of attaining equilibrium states in silicate systems. The existence in the $\mathrm{BaO}$—$\mathrm{SiO_2}$ system ($^{12}$) of the compound $2\mathrm{BaO}\cdot 3\mathrm{SiO_2}$, which forms continuous solid solutions with $\mathrm{BaSi_2O_5}$, and the absence of a compound analogous in formula in the $\mathrm{RbF}$—$\mathrm{BeF_2}$ system requires verification of the individuality of the compound $2\mathrm{BaO}\cdot 3\mathrm{SiO_2}$. It is possible that $2\mathrm{BaO}\cdot 3\mathrm{SiO_2}$ represents a case of a limiting solid solution of $\mathrm{BaSiO_3}$ in $\mathrm{BaSi_2O_5}$—of composition close in stoichiometry to $2\mathrm{BaO}\cdot 3\mathrm{SiO_2}$.

In conclusion, I express my deep gratitude to Prof. N. A. Toropov for his scientific guidance of the work.

Institute of Silicate Chemistry
Academy of Sciences of the USSR

Received
15 XI 1956

REFERENCES CITED

1. E. Thilo, H. Lehmann, Zs. anorg. Chem., 258, 3—5, 332 (1949).
2. A. V. Novoselova, Yu. P. Simanov, E. I. Yarembash, ZhFKh, 26, 9, 1245 (1952).
3. E. Thilo, H. Schröder, Zs. phys. Chem., 197, 1—2, 39 (1951).
4. D. M. Roy, R. Roy, E. F. Osborn, J. Amer. Ceram. Soc., 36, 6, 185 (1953).
5. A. V. Novoselova, M. E. Levina, Yu. P. Simanov, A. G. Zhukmin, ZhOKh, 14, 6, 385 (1944).
6. P. Mukherjie, Ind. J. Phys., 18, 3, 148 (1944).
7. A. V. Novoselova, Yu. P. Simanov, Uch. Zap. MGU, issue 174, 7 (1955).
8. N. A. Toropov, R. G. Grebenshchikov, Zhurn. neorg. khim., 1, 7, 1619 (1956).
9. N. A. Toropov, R. G. Grebenshchikov, Zhurn. neorg. khim., 1, 12, 2686 (1956).
10. A. V. Novoselova, M. Ya. Averkova, Zhurn. neorg. khim., 9, 12, 1063 (1939).
11. D. F. Kirkina, A. V. Novoselova, Yu. P. Simanov, DAN, 107, No. 6, 837 (1956).
12. P. Eskola, Am. J. Sci. (4), 4, 23, 331 (1922).