CHEMISTRY

R. G. GREBENSHCHIKOV, Corresponding Member of the Academy of Sciences of the USSR N. A. TOROPOV, V. I. SHITOVA

CRYSTALLINE PHASES OF THE SYSTEM BARIUM OXIDE—GERMANIUM DIOXIDE

Like fluoroberyllates, which in the light of V. M. Goldschmidt’s concepts are crystallochemical models of silicates, germanates likewise show a similarity of crystalline structure to silicates. However, in contrast to fluoroberyllates, germanates and silicates, owing to the closeness of their principal physical properties, form broad series of isomorphous mixtures with one another. Solid solutions based on germanates and silicates are an example of isomorphous substitutions in the anionic part of the lattice (^1, ^2).

Table 1

The system BaO—GeO₂ may to a certain extent be regarded as an analog of the MeO—SiO₂ systems, where Me²⁺—Ca, Sr, Ba. Therefore the study of germanates is of definite interest for silicate technology and, in particular, for the chemistry of cements and other mineral binders. At present the literature contains only fragmentary information on the syntheses and investigations of the properties of barium germanates (^3–^6). In order to fill this existing gap, we studied the conditions of synthesis and the principal properties of germanates in the system BaO—GeO₂.

For the preparation of barium germanates the reagents used were: BaCO₃ (analytical grade) and GeO₂ (chemically pure grade). Samples of barium germanates, pressed in the form of tablets, were subjected to repeated firing with intermediate grinding. In all, eight compositions with different BaO : GeO₂ ratios were studied: 4 : 1, 3 : 1, 2 : 1, 3 : 2, 2 : 3, 1 : 2, 1 : 3, and 1 : 4. The firing temperature of samples with BaO : GeO₂ ratios of 4 : 1, 3 : 1, and 2 : 1 was raised to 1450° with a holding time of about 3 hr. Compositions with BaO : GeO₂ ratios of 3 : 2 and 2 : 3 were fired at 1200°, the 1 : 2 composition at 1150°, and the 1 : 3 and 1 : 4 compositions at 1250° and 1350°, respectively.

Table 2

According to X-ray (at ordinary and high temperatures), crystallo-optical in immersion high-refractive-index phosphorus liquids, and differential-thermal analyses, four individual compounds of the following composition were found in the system BaO—GeO₂: 3BaO·GeO₂, 2BaO·GeO₂, BaO·GeO₂, and BaO·4GeO₂. Tribarium germanate is a new chemical compound, about which there is no information in the literature. Table 1 gives the results of chemical analysis of the barium germanates; Table 2 gives the light refraction, measured with an accuracy of ±0.005, and the densities of the barium germanates, determined in outgassed kerosene at 25°.

In the homologous series of barium germanates (see Table 2), the density and refractive index, beginning with the highly basic compound \(3\mathrm{BaO}\cdot\mathrm{GeO}_2\), decrease to the metagermanate \(\mathrm{BaO}\cdot\mathrm{GeO}_2\) and then increase again in the tetragermanate \(\mathrm{BaO}\cdot4\mathrm{GeO}_2\); in this, the minimum values of density and refractive index, which occur for \(\mathrm{BaGeO}_3\), correspond to the maximum birefringence of the metagermanate. Such an “anomalous” change in properties in the homologous series of barium germanates is most probably explained by the presence in \(\mathrm{BaGe}_4\mathrm{O}_9\)—the crystallochemical analogue of benitoite \(\mathrm{BaTiSi}_3\mathrm{O}_9\)—of octahedral coordination for \(1/4\) of the formal germanium content in the tetragermanate. This leads to densification of the \(\mathrm{BaGe}_4\mathrm{O}_9\) structure and, correspondingly, to an increase in its density and refractive index as compared with the metagermanate. In the germanates \(\mathrm{Ba}_3\mathrm{GeO}_5\), \(\mathrm{Ba}_2\mathrm{GeO}_4\), and \(\mathrm{BaGeO}_3\), the germanium cations occur only in tetrahedral coordination.

Fig. 1. Thermogram of tribarium germanate \(3\mathrm{BaO}\cdot\mathrm{GeO}_2\)

According to X-ray and thermal investigations, \(\mathrm{Ba}_3\mathrm{GeO}_5\) is stable in the solid state in the temperature interval \(20\)—\(1500^\circ\). The thermogram of \(\mathrm{Ba}_3\mathrm{GeO}_5\), as is evident from Fig. 1, indicates the presence of modification transformations; this is also confirmed by X-ray patterns of tribarium germanate taken in the temperature range from room temperature to \(1300^\circ\), see Fig. 2. On quenching, the high-temperature forms of \(\mathrm{Ba}_3\mathrm{GeO}_5\) without addition of mineralizers are practically not preserved; the specimens disintegrate. The complexity of the scheme of modification transformations of \(\mathrm{Ba}_3\mathrm{GeO}_5\) requires an independent study of its \(P\)—\(T\) diagram, of the structures of the polymorphic varieties, and also of the influence of mineralizers on the stabilization of the individual forms of \(\mathrm{Ba}_3\mathrm{GeO}_5\).

Fig. 2. X-ray powder patterns of \(3\mathrm{BaO}\cdot\mathrm{GeO}_2\) and of zeolite \(\mathrm{BaO}\cdot3.5\mathrm{GeO}_2\cdot n\mathrm{H}_2\mathrm{O}\), taken with CuK\(\alpha\) radiation at various temperatures

Barium orthogermanate \(\mathrm{Ba}_2\mathrm{GeO}_4\), according to X-ray and thermal investigations, shows no modification transformations up to \(1300^\circ\).

Barium metagermanate \(\mathrm{BaGeO}_3\) crystallizes in two polymorphic varieties: pyroxenoid—with a chain germanate–oxygen radical, and pseudowollastonite—with a ring radical \((^3,^6)\). In our investigation, in synthesis in the solid phase, chain \(\mathrm{BaGeO}_3\) was obtained; it showed no modification transformations up to \(1100^\circ\).

In contrast to the \(\mathrm{BaO}—\mathrm{SiO}_2\) system \((^7,^8)\), in which the individual compounds \(2\mathrm{BaO}\cdot3\mathrm{SiO}_2\) and \(\mathrm{BaO}_2\cdot2\mathrm{SiO}_2\) exist, in the \(\mathrm{BaO}—\mathrm{GeO}_2\) system no germanates with a similar stoichiometric composition were found. At the same time, in the \(\mathrm{BaO}—\mathrm{GeO}_2\) system \(\mathrm{BaO}\cdot4\mathrm{GeO}_2\) was obtained, which has no stoichiometric analogue in the silicate system. Barium tetragermanate shows no polymorphism up to its melting point, \(1390^\circ\).

In accordance with the procedure described in work \((^9)\), we synthesized a barium–germanate zeolite of composition \(\mathrm{BaO}\cdot3.5\mathrm{GeO}_2\cdot n\mathrm{H}_2\mathrm{O}\), which in the pro-

during heating undergoes a series of endothermic structural transformations caused by dehydration of the zeolite. At a temperature of about 700° barium tetragermanate is formed from the dehydrated product of zeolite composition; in this case the reaction proceeds with a very large exothermic effect. Fig. 2 gives X-ray powder photographs of the zeolite obtained during its heating; the X-ray photograph at 760° corresponds to BaO·4GeO₂—the final member of the modification series of transformations of the zeolite.

Fig. 3. X-ray powder line diagrams of crystalline phases of the BaO—GeO₂ system:
3BaO·GeO₂, 2BaO·GeO₂, BaO·GeO₂, and BaO·4GeO₂.

For a more complete characterization of the synthesized germanates, Fig. 3 gives X-ray powder line diagrams (with values of interplanar spacings) of the low-temperature form Ba₃GeO₅, Ba₂GeO₅, chain BaGeO₃, and BaGe₄O₉.

In conclusion, we note that among the synthesized germanates the triebarium germanate Ba₃GeO₅ is of the greatest interest—a crystal-chemical analogue of the principal product of cement clinker, Ca₃SiO₅. Investigation of the structure of the polymorphic varieties of Ba₃GeO₅ and of its hydraulic activity is of great importance for solving many problems in the physical chemistry of cement silicates.

Institute of Silicate Chemistry named after I. V. Grebenshchikov
Academy of Sciences of the USSR

Received
3 V 1963

CITED LITERATURE

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