PHYSICAL CHEMISTRY

R. G. GREBENSHCHIKOV and N. A. TOROPOV

PHASE DIAGRAM OF THE SYSTEM BaO—SiO₂ IN THE REGION OF ELEVATED SILICA CONTENT

(Presented by Academician N. V. Belov, July 14, 1961)

The system BaO—SiO₂, studied as early as 1922 by P. Eskola (¹), is important for silicate technology. At present it has entered, without changes, into many ternary systems: Li₂O, CaO, B₂O₃, Al₂O₃/SiO₂—BaO (², ³). Contradictions in the proposed versions of the equilibrium relations of the components in the Ba₂Si₃O₈—BaSi₂O₅ region of the barium oxide—silica system required further study of this region. Among systems Me²⁺O—SiO₂, only in the case of BaO—SiO₂ have compounds of the stoichiometry 2MeO·3SiO₂ and MeO·2SiO₂ been found: dibarium trisilicate Ba₂Si₃O₈ (B₂S₃) and barium disilicate BaSi₂O₅ (BS₂). On the basis of a crystal-optical study, P. Eskola represented the solid-phase relations of the components in the B₂S₃—BS₂ region as involving the formation of continuous solid solutions. Simultaneously with our study of the B₂S₃—BS₂ region, an analogous investigation by R. Roth and E. Levin (⁴) refuted the existence of solid solutions; instead, in this interval two new compounds were found: 5BaO·8SiO₂ (B₅S₈)—congruently melting, and 3BaO·5SiO₂ (B₃S₅)—incongruently melting, with closely similar refractive-index values.

The solution of the question of the nature of the phases in the B₂S₃—BS₂ region was facilitated by our obtaining a number of data on the structures of BaSiO₃, Ba₂Si₃O₈, and BaSi₂O₅ (⁵–⁸). In our work (⁸), on the basis of X-ray, spectroscopic, and crystallochemical investigations of BS, B₂S₃, and BS₂, it was shown that the structures of B₂S₃ and BS₂ can be derived by condensation of the chains \([Si_2O_6]_{\infty}^{4-}\)—an element of the structure of barium metasilicate: 1) into triple chains (ribbons) with an anionic radical \(3 \times [Si_2O_6]_{\infty}^{4-} - 2O = [Si_6O_{16}]_{\infty}^{8-}\)—for B₂S₃* and 2) into layers with the radical \([Si_4O_{10}]^{4-}\)—for BS₂ (sanbornite). In the lattices of BS, B₂S₃, and BS₂ one of the parameters, 4.6 kX, corresponding to the repeat period of the metasilicate chain \([Si_2O_6]_{\infty}^{4-}\), remains unchanged upon condensation of the chains in the structures of B₂S₃ and BS₂.

For constructing the BaO—SiO₂ diagram in the composition interval from 58 to 67 mol.% SiO₂, B₂S₃ and BS₂ were synthesized from starting BaCO₃ (analytical grade) and amorphous silica (chemically pure) by repeated firing (not fewer than three times) at 1400° (the holding time at the maximum temperature was 5–6 hr), with intermediate grinding. The homogeneity of the finished fired products B₂S₃ and BS₂ was checked in each case microscopically. Compositions corresponding to the proposed compounds B₅S₈ and B₃S₅ were obtained in two ways: from starting BaCO₃ and amorphous silica, and from the finished B₂S₃ and BS₂. The identity of each of these products, obtained by different—

* Needle-shaped single crystals of B₂S₃ were obtained by prolonged crystallization from a melt. From X-ray photographs of B₂S₃ rotation about the MoKα needle axis, the identity period 4.76 kX was calculated, which corresponds to the repeat period of the silicic-acid ribbon in the B₂S₃ lattice.

by methods that confirmed this, by X-ray diffraction and crystal-optical methods. In the concentration range studied, six compositions were prepared from the starting materials \(\mathrm{BS}_2\) and \(\mathrm{BaCO}_3\):

\[ \mathrm{Ba}_2\mathrm{Si}_4\mathrm{O}_{10} + 2/15,\ 4/15,\ 6/15,\ 8/15,\ 10/15\ (\text{corresponds to } \mathrm{B}_2\mathrm{S}_3) \]

and \(12/15\) mol.% \(\mathrm{BaO}\). These compositions were fired twice, with intermediate grinding, at temperatures close to the onset of melting of the samples.

Characterization of the phase composition of the samples was carried out by X-ray structural and crystal-optical analysis. All samples were recorded in VRS-3 cameras using MoK\(_\alpha\) radiation and on an ionization apparatus in the temperature range \(20\)—\(1400^\circ\) using CuK\(_\alpha\) radiation. The optical study was carried out in monochromatic light of the sodium \(D\)-line.

Fig. 1. Dependence of the refractive index of barium silicates on composition in the range 58.5—66.67 mol.% \(\mathrm{SiO}_2\). 1 — solid lines — our data; 2 — points \(a\) — data \((^1)\); 3 — points \(b\) correspond to the refractive indices of \(\mathrm{B}_5\mathrm{S}_8\) and \(\mathrm{B}_3\mathrm{S}_5\) according to \((^4)\).

The solid-phase equilibrium data for annealed samples (under normal conditions) in the concentration range from \(\simeq 58.5\) to 66.67 mol.% \(\mathrm{SiO}_2\) are illustrated by the graph shown in Fig. 1, giving the change in refractive indices \(N_g^D\) and \(N_p^D\) as a function of their composition. For samples of composition \(\simeq 58.5\)—62.5%, a smooth change in refractive index is observed from \(N_g^D = 1.650\) and \(N_p^D = 1.630\) to \(N_g^D = 1.631\) and \(N_p^D = 1.612\), respectively. Under the microscope, the phases of variable composition are represented by a homogeneous product in the form of birefringent needles with straight extinction (in some cases oblique extinction with an angle up to \(5^\circ\) was observed), distinct cleavage along the elongation axis of the crystallites, and \((—)\) elongation. In samples of composition 62.5—66.67%, two phases were observed: a solid solution \(\mathrm{B}_2\mathrm{S}_3\) of limiting composition with stoichiometry close to \(\mathrm{B}_3\mathrm{S}_5\), and barium disilicate \(\mathrm{BS}_2\). Therefore, for these compositions the change in the upper refractive index is expressed by a straight line parallel to the abscissa axis, with constant \(N_g^D = 1.631\), corresponding to the solid solution.

Fig. 2. Phase diagram of the \(\mathrm{BaO}\)—\(\mathrm{SiO}_2\) system in the composition range 58—68 mol.% \(\mathrm{SiO}_2\).

The value of the lower refractive index in this concentration range also retains the constant value \(N_p^D = 1.596\), corresponding to \(\mathrm{BS}_2\). A sharp change in birefringence (Fig. 1) at the composition boundary 62.5% was also observed by P. Eskola; however, he did not interpret it as being associated with the appearance of two phases in this range.

The phase diagram of the \(\mathrm{BaO}\)—\(\mathrm{SiO}_2\) system shown in Fig. 2 const-

constructed chiefly on the basis of a comprehensive study of the relationships of the components in the solid phase. The use of classical methods of physicochemical analysis, based on study of the character of the liquidus curve (its breaks), in the present case could not be considered justified for the following reasons: the close melting temperatures of $\mathrm{B_2S_3}$ ($1447^\circ$) and $\mathrm{BS_2}$ ($1420^\circ$) and the narrow range of variation of $\mathrm{SiO_2}$ concentrations in the presence of phases of variable composition, with their inevitable structural zonality, complicated the experiment and could not guarantee the attainment of equilibrium crystallization of phases from the melt over short intervals of time. The latter was a necessary condition for maintaining the constancy of the composition of the samples. Therefore the firing of the samples was carried out in the subsolidus region at temperatures close to the boundary lines of the solidus.

In the investigated interval of the system, the maximum on the liquidus curve at $1447^\circ$ corresponds to the compound $\mathrm{B_2S_3}$. The region of phases of variable composition formed on the basis of $\mathrm{B_2S_3}$ extends from $\simeq 58.5$ to $62.5$ mol. %. Annealing of $\mathrm{B_2S_3}$ samples with an elevated $\mathrm{BaO}$ content in the composition range $\simeq 58.5$—$60.0$ mol. % $\mathrm{SiO_2}$ at $1300^\circ$ lowers the solubility of $\mathrm{BaO}$ in $\mathrm{B_2S_3}$, which is shown on the diagram by the slope of the solubility boundary of $\mathrm{BaO}$. The solubility of $\mathrm{BS_2}$ in $\mathrm{B_2S_3}$ is limited to $62.5$ mol. % $\mathrm{SiO_2}$. Two polymorphic modifications were found for barium disilicate: the low-temperature form $\beta$-$\mathrm{BS_2}$, stable under normal conditions and identical with the natural mineral sanbornite; at $1350^\circ$ $\beta$-$\mathrm{BS_2}$ transforms into the high-temperature form $\alpha$-$\mathrm{BS_2}$, stable up to $1420^\circ$ (melting). On the diagram the boundary line at $1350^\circ$ separates the stability fields of $\beta$- and $\alpha$-$\mathrm{BS_2}$. The solubility of $\mathrm{BaO}$ in $\alpha$-$\mathrm{BS_2}$ increases with increasing temperature, reaching a limit at the temperature of the peritectic reaction ($1425^\circ$). In the low-temperature form $\beta$-$\mathrm{BS_2}$, $\mathrm{BaO}$ is practically insoluble.

Simultaneous holding at $1415^\circ$, i.e., somewhat below the melting of barium disilicate, of samples $\mathrm{BS_2}$ and $\mathrm{B_2S_4} + 2/15,\ 4/15,\ \ldots\ 12/15$ mol. % $\mathrm{BaO}$ (previously fired twice) was not accompanied by their melting, whereas raising the temperature by $\simeq 10^\circ$ caused considerable melting of $\mathrm{BS_2}$, $\mathrm{B_2S_4} + 2/15$ mol. % $\mathrm{BaO}$ ($65.2$ mol. % $\mathrm{SiO_2}$), and $\mathrm{B_2S_4} + 4/15$ mol. % $\mathrm{BaO}$ ($63.8$ mol. % $\mathrm{SiO_2}$), and only slight melting of the sample $\mathrm{B_2S_4} + 6/15$ mol. % $\mathrm{BaO}$ ($62.45$ mol. % $\mathrm{SiO_2}$). The melting of the indicated samples, proceeding from the established equilibrium in the solid phase, can be associated only with the peritectic reaction
$\mathrm{B_2S_{3\,tr}} \rightleftarrows \mathrm{BS_{2\,tr}} + \text{liq.}$, to which the boundary line at $1425^\circ$ corresponds on the diagram.

It should be noted that a certain uncertainty in the question of the structure of the phases of variable composition $\mathrm{B_2S_3}$ does not at present make it possible to assert unambiguously the continuity of the change in composition of $\mathrm{B_2S_3}$ in the interval $\simeq 58.5$—$62.5\%$. The similarity of the crystal structure of $\mathrm{B_2S_3}$ and its solid solutions follows from the similarity of the X-ray diffraction patterns shown in Fig. 3. At the same time, the differing change in interplanar spacings of identical reflections is caused by different changes in the three parameters of the rhombic solid solutions of $\mathrm{B_2S_3}$ as a function of composition. X-ray ionization powder patterns of the solid solutions of $\mathrm{B_2S_3}$ and $\mathrm{BS_2}$ show a considerable difference between them.

The formation of the numerous group of barium silicates and, in particular, $\mathrm{B_2S_3}$—the sole representative of such stoichiometry among the silicates of alkali and alkaline-earth metals—is explained by the pronounced tendency toward condensation of silico-oxygen anions with change in the ratio $\mathrm{BaO} : \mathrm{SiO_2}$ in barium silicates. In this connection it is possible that the phases of variable composition formed on the basis of $\mathrm{B_2S_3}$ may represent products of further condensation of the ribbons $[\mathrm{Si_6O_{16}}]_\infty^{8-}$ and chains $[\mathrm{Si_2O_6}]_\infty^{4-}$. For example, the anionic radicals of compositions corresponding to the stoichiometry of the assumed compounds $\mathrm{B_5S_8}$ and $\mathrm{B_3S_5}$ may be represented as condensations: 1) $[\mathrm{Si_6O_{16}}]_\infty^{8-} + [\mathrm{Si_2O_6}]_\infty^{4-} - \mathrm{O} = [\mathrm{Si_8O_{21}}]_\infty^{10-}$—for $\mathrm{B_5S_8}$ and 2) $[\mathrm{Si_6O_{16}}]_\infty^{8-} + 2[\mathrm{Si_2O_6}]_\infty^{4-} - 2\mathrm{O} = [\mathrm{Si_{10}O_{26}}]_\infty^{12-}$—for $\mathrm{B_3S_5}$. Such

the character of the complication of the anionic radicals suggests condensation of chains and ribbons in one layer, but other modes are also possible.

The noted break in solubility in the region $\mathrm{B_2S_3}$—$\mathrm{BS_2}$, with the formation of a narrow range of phases of variable composition, is caused by the attainment, for the ribbon structure, of the condensation limit equal to 5 metasilicate chains

Fig. 3. Schematic representation of X-ray powder ionization diagrams recorded with CuK$\alpha$ radiation. Compositions: $a$ — $\mathrm{B_2S_3}$ with addition of 5 wt.% BaO, $б$ — $\mathrm{B_2S_3}$, $в$ — $\mathrm{B_5S_8}$, $г$ — $\mathrm{B_3S_5}$, $д$ — $\beta$-$\mathrm{BS_2}$ (sanbornite)

chains (composition $\mathrm{B_3S_5}$). Consequently, phases of composition $\mathrm{B_2S_3}$, $\mathrm{B_5S_8}$, $\mathrm{B_3S_5}$ may be regarded not only as members of one series of solid solutions, but also as distinct products of condensation of metasilicate chains. The high hardness of the $\mathrm{B_2S_3}$—$\mathrm{BS_2}$ silicates (of the order of 6–7), together with low coefficients of thermal expansion, will make it possible on their basis to obtain, for example in the $\mathrm{BaO}$—$\mathrm{Al_2O_3}$—$\mathrm{SiO_2}$ system, new glass-crystalline materials opaque to $\gamma$-radiation.

Institute of Silicate Chemistry
Academy of Sciences of the USSR

Received
14 VII 1961

CITED LITERATURE

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