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CHEMISTRY
Corresponding Member of the Academy of Sciences of the USSR N. A. TOROPOV, A. K. SHIRVINSKAYA
SOLID SOLUTIONS IN THE SYSTEM Ca₂SiO₄—Ca₂GeO₄
The crystal-chemical similarity between calcium orthosilicate and orthogermanate was established by Shtrunts and Jakob (1). A further analogy with respect to the physicochemical properties of these compounds, and also between Ca₃SiO₅ and Ca₃GeO₅, was revealed in (2, 3). Therefore, a physicochemical investigation of calcium germanates as analogues of calcium silicates is of interest for the chemistry of cements, refractories, etc.
Dicalcium germanate, according to (3), crystallizes in two polymorphic forms. The low-temperature variety of Ca₂GeO₄ has an olivine-type structure (1) and will hereafter be denoted by us as l.t. (low-temperature) Ca₂GeO₄. The high-temperature form of Ca₂GeO₄ is stable above 1450° and, according to X-ray analysis (the most characteristic lines: 3.27, 2.94, 2.82, 2.71, 2.17, 1.97, 1.73, 1.60, 1.57, 1.49), is completely identical with α-Ca₂SiO₄. Hence one may expect the existence of isomorphism between dicalcium silicate and dicalcium germanate. The study of this question is of interest in light of the elucidation of anionic isomorphism for Ca₂SiO₄, on which only limited data are available in the literature (4–8). For this purpose we studied solid solutions between dicalcium silicate and dicalcium germanate.
Fig. 1. Dependence of the birefringence for annealed specimens in the system Ca₂SiO₄—Ca₂GeO₄ on their composition.
To study the phase equilibrium of the system, 23 compositions were prepared. The starting materials used for preparing the mixtures were CaCO₃ (chemically pure), crystalline SiO₂ (99.9%), and amorphous GeO₂ (99.9%) in one series of experiments, and γ-Ca₂SiO₄ and l.t. Ca₂GeO₄ in another. It should be noted at once that the results of the experiments for both series were identical. Carefully mixed batches were pressed in the form of cylinders, and the latter were fired in a silite furnace at a temperature of about 1500°. After the first firing, mixtures containing more than 60 mol.% and less than 40 mol.% Ca₂GeO₄ were homogeneous, while mixtures with intermediate Ca₂GeO₄ content consisted of two phases. After additional firings with intermediate grinding, homogeneity was achieved for all compositions. After firing, the specimens were cooled slowly together with the furnace; in this process all compositions containing from 0 to 42.5% Ca₂GeO₄ crumbled into powder, whereas specimens with a higher Ca₂GeO₄ content retained the shape of sintered compacts.
The synthesized products were investigated by microscopic, X-ray, and differential thermal analyses. X-ray recording was carried out on a URS-50I ionization unit using CuKα radiation at a rate of 1°/min.
As a result of the microscopic investigation, the existence of two single-phase regions was established in the system. Compositions containing from 0 to 42.5% Ca₂GeO₄ are represented by elongated, weakly birefringent
prismatic crystals with straight extinction, positive elongation, and strongly pronounced twinning along the elongation axis. Compositions with a $\mathrm{Ca_2GeO_4}$ content from 43.75 to 100% were observed in the form of rounded or fragmental crystals with much higher birefringence. The change in refractive indices for the synthesized samples as a function of their composition is shown in Fig. 1. The graph shows
Fig. 2. X-ray patterns of solid solutions: a, б, в—$\gamma$-$\mathrm{Ca_2SiO_4}$ solid solution; a—$\gamma$-$\mathrm{Ca_2SiO_4}$; б—60% $\mathrm{Ca_2SiO_4}$ + 40% $\mathrm{Ca_2GeO_4}$; в—57.5% $\mathrm{Ca_2SiO_4}$ + 42.5% $\mathrm{Ca_2GeO_4}$; г, д, е—low-temperature $\mathrm{Ca_2GeO_4}$ solid solution; г—55% $\mathrm{Ca_2SiO_4}$ + 45% $\mathrm{Ca_2GeO_4}$; д—40% $\mathrm{Ca_2SiO_4}$ + 60% $\mathrm{Ca_2GeO_4}$; е—low-temperature $\mathrm{Ca_2GeO_4}$.
a uniform increase in the refractive indices from $n_p = 1.644$ and $n_g = 1.652$ to $n_p = 1.676$ and $n_g = 1.680$; then, after a sharp increase in birefringence, a uniform increase in $n_p$ and $n_g$ is again observed, from values of 1.674 and 1.692 to 1.700 and 1.734, respectively.
These data are in complete agreement with the data of X-ray phase analysis. Fig. 2 presents X-ray patterns of several compositions in the $\mathrm{Ca_2SiO_4}$—$\mathrm{Ca_2GeO_4}$ system. For compositions containing $\mathrm{Ca_2GeO_4}$ in the range from 0 to 42.5%, the X-ray pattern of $\gamma$-$\mathrm{Ca_2SiO_4}$ is retained, whereas compositions with a higher $\mathrm{Ca_2GeO_4}$ content show the X-ray pattern of low-temperature $\mathrm{Ca_2GeO_4}$. Because the values of the interplanar spacings for $\gamma$-$\mathrm{Ca_2SiO_4}$ and low-temperature $\mathrm{Ca_2GeO_4}$ are close, the transition from one crystalline form to the other is not so visually apparent, but it is quite definite, as can be seen from the sharp change in intensities of the strongest lines and from a certain general change in the character of the X-ray patterns. In the transition from $\gamma$-$\mathrm{Ca_2SiO_4}$ to low-temperature $\mathrm{Ca_2GeO_4}$, a uniform shift of the lines toward smaller angles is observed.
Thus, on the basis of microscopic and X-ray phase analyses, the existence of two types of solid solutions has been established in the low-temperature part of the system: one solid solution based on $\gamma$-$\mathrm{Ca_2SiO_4}$ within the range of $\mathrm{Ca_2GeO_4}$ content from 0 to 42.5%, and a second solid solution based on low-temperature $\mathrm{Ca_2GeO_4}$ when its content is from 43.75 to 100%.
High-temperature phase equilibrium was studied by means of high-temperature X-ray photography, a high-temperature microscope, and thermal analysis. The data of differential thermal analysis are presented in Fig. 3. Samples containing from 0 to 38% $\mathrm{Ca_2GeO_4}$ show on the thermograms two endothermic effects, which correspond to the transition of the $\gamma$-$\mathrm{Ca_2SiO_4}$ solid solution into the high-temperature $\alpha'$- and $\alpha$-modifications.
The region of the $\gamma$-$\mathrm{Ca_2SiO_4}$ solid solution between 38.0 and 42.5% passes, on heating, directly into the $\alpha$-solid solution, bypassing the $\alpha'$-form. On the thermograms of samples for this part of the system, one large endothermic effect is observed.
Samples of the low-temperature $\mathrm{Ca_2GeO_4}$ solid solution show one endothermic effect, corresponding to the polymorphic transformation into the high-temperature $\alpha$-form of the solid solution. The characterization of the phase composition in the high-temperature part of the diagram is confirmed by high-temperature X-ray photography data.
On the basis of the data obtained, a phase diagram of the $\mathrm{Ca_2SiO_4}$—$\mathrm{Ca_2GeO_4}$ system was constructed, presented in Fig. 4. In the low-temperature part of the diagram there exist two solid solutions: a $\gamma$-$\mathrm{Ca_2SiO_4}$ solid solution from 0 to 42.5% and a low-temperature $\mathrm{Ca_2GeO_4}$ solid solution from 43.75 to 100% $\mathrm{Ca_2GeO_4}$. We were unable to establish a two-phase region within the investigated compositions, although the gap between neighboring compositions of the two solid solutions was 1.25 mole %. Apparently, this region exists in a very narrow concentration interval. On the diagram this region is indicated by dashed lines.
The boundary curves between the different polymorphic forms of the solid solutions were constructed from the temperatures of the endothermic minima corresponding to the respective polymorphic transformations.
Fig. 3. Differential thermal curves of solid solutions in the $\mathrm{Ca_2SiO_4}$—$\mathrm{Ca_2GeO_4}$ system
The positions of the lower boundary curves were not determined, and they are drawn hypothetically. The solubility of $\mathrm{Ca_2GeO_4}$ in the $\alpha'$-$\mathrm{Ca_2SiO_4}$ solid solution is $\simeq 37\%$. In the high-temperature part of the diagram, a single-phase solid-solution region is realized between $\alpha$-$\mathrm{Ca_2SiO_4}$ and $\alpha$-$\mathrm{Ca_2GeO_4}$.
On cooling, both during annealing and during quenching in air, the high-temperature forms of the solid solutions transform into the low-temperature $\gamma$-$\mathrm{Ca_2SiO_4}$ and $\mathrm{Ca_2GaO_4}$ solid solutions. High-temperature X-ray photography made it possible to establish that, on cooling, samples with a small content of $\mathrm{Ca_2GeO_4}$ (up to 15%) pass through a metastable-
… the $\beta$-$\mathrm{Ca_2SiO_4}$ form of the solid solution, whereas for specimens with a higher content of $\mathrm{Ca_2GeO_4}$ (up to 42.5%) the $\alpha'$-$\mathrm{Ca_2SiO_4}$ form of the solid solution becomes metastable.
The slope of the liquidus curve was determined from the melting temperatures of several compositions relative to the melting temperatures of the initial $\mathrm{Ca_2SiO_4}$ and $\mathrm{Ca_2GeO_4}$, using a high-temperature microscope. The design of the latter and the procedure for working with it are described in (9). Since direct measurement of the temperature was not carried out, the liquidus curve is drawn as a dashed line. The melting temperature of $\mathrm{Ca_2GeO_4}$ is $1900 \pm 30^\circ$.
Fig. 4. Phase diagram of the system $\mathrm{Ca_2SiO_4}$—$\mathrm{Ca_2GeO_4}$.
$a$—compositions investigated, $b$—temperatures of endothermic minima, $c$—high-temperature X-ray photography.
As a result of the study of solid solutions between $\mathrm{Ca_2SiO_4}$ and $\mathrm{Ca_2GeO_4}$, a phase diagram of the system $\mathrm{Ca_2SiO_4}$—$\mathrm{Ca_2GeO_4}$ was constructed, in which the existence of four stable and one metastable forms of solid solutions was established.
Institute of Silicate Chemistry
named after I. V. Grebenshchikov
Academy of Sciences of the USSR
Received
26 VII 1963
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