CHEMISTRY

B. I. MARKHASEV, I. D. SEDLETSKII

ON SOME REGULARITIES IN THE STRUCTURE OF MELTS IN THE SYSTEMS $\mathrm{Me}_2\mathrm{O}_3$—$\mathrm{SiO}_2$

(Presented by Academician N. V. Belov, September 13, 1963)

It was shown earlier that the basic principles of crystal chemistry can be successfully applied in considering the structure of silicate melts ($^1$). In particular, the presence of two immiscible phases was related to the mismatch between the dimensions of the Me cations and the position assigned to the silicon cation by the oxygen framework. It was noted that, as the content of

Fig. 1. Liquidus lines of the systems $\mathrm{Me}_2\mathrm{O}_3$—$\mathrm{SiO}_2$

1. $\mathrm{Sc}_2\mathrm{O}_3$—$\mathrm{SiO}_2$
2. $\mathrm{Al}_2\mathrm{O}_3$—$\mathrm{SiO}_2$
3. $\mathrm{Cr}_2\mathrm{O}_3$—$\mathrm{SiO}_2$
4. $\mathrm{La}_2\mathrm{O}_3$—$\mathrm{SiO}_2$
5. $\mathrm{Nd}_2\mathrm{O}_3$—$\mathrm{SiO}_2$

$\mathrm{SiO}_2$ in the melt increases, the oxygen coordination of the metal cations increases. Below, these principles are applied in considering certain features of the structure of melts in the systems $\mathrm{Me}_2\mathrm{O}_3$—$\mathrm{SiO}_2$.

In Fig. 1 the liquidus lines of a number of systems are plotted from the data of ($^{2-4}$). The corresponding numerical values are given in Table 1. The ionic radii of the cations were calculated taking into account the type of coordination and the magnitude of the Born repulsion coefficient according to the data of ($^5$). The last column of the table gives the values of the maximum oxygen coordination, determined from the diagram in ($^1$). In all the systems considered, with the exception—

In systems of the type Al₂O₃—SiO₂, two-phase regions of coexistence of two liquid melts are observed, one of which is close in composition to SiO₂. The composition of the second melt, enriched in Me₂O₃, as shown by the data in Table 1 and Fig. 2, depends on the size of the metal cation. As the radius decreases, the size restrictions that hinder the entry of the cation into the silicate framework increase, and the region of immiscibility of the silicate melt expands. A particularly sharp broadening of the immiscibility interval occurs on going from scandium \((R_{\mathrm{Sc}} = 0.78\ \text{Å})\) to chromium \((R_{\mathrm{Cr}} = 0.64\ \text{Å})\). The same broad immiscibility interval is observed in the TiO₂—SiO₂ system \((R_{\mathrm{Ti}} = 0.66\ \text{Å})\) (Fig. 2). The Nd₂O₃—SiO₂ system deviates from the general regularity (Fig. 2). The immiscibility interval in this system is somewhat wider than might have been expected on the basis of the value of the ionic radius. At present it does not appear possible to give a rational explanation for this deviation.

Fig. 2. Content of SiO₂ in the melt maximally enriched in Me₂O₃, as a function of cation radius

The absence of immiscibility in melts of the Al₂O₃—SiO₂ system is explained by the fact that the aluminum cation can occur in fourfold oxygen coordination, forming tetrahedral groups isomorphous with silica tetrahedra. The possibilities for isomorphous replacement of silicon by aluminum, observed also in crystalline phases, are broadened in the molten state owing to a certain statistical smearing of the atomic positions, which is inherent in a liquid. In this connection, the restrictions on the placement of cations in the silicate framework in the Al₂O₃—SiO₂ system are removed.

A comparison of the widths of the immiscibility intervals in melts of silicates of divalent and trivalent metals (Fig. 2) shows that, for the same value of \(R_k\), the immiscibility interval in the Me₂O₃—SiO₂ systems proves to be narrower. This may be explained by the stronger polarizing action exerted by trivalent cations on oxygen ions, which somewhat increases the value of the maximum possible oxygen coordination.

Table 1

It is interesting to note an analogous relationship between cation sizes and the possibilities of their existence in silicate groups in crystalline phases. In systems formed by metals with appreciable values of \(R_k\) (La₂O₃—SiO₂, Nd₂O₃—SiO₂), there are three crystalline phases \((\mathrm{Me}_2\mathrm{O}_3 \cdot \mathrm{SiO}_2;\ 2\mathrm{Me}_2\mathrm{O}_3 \cdot 3\mathrm{SiO}_2;\ \mathrm{Me}_2\mathrm{O}_3 \cdot 2\mathrm{SiO}_2)\); in the Sc₂O₃—SiO₂ system there are two crystalline phases \((\mathrm{Sc}_2\mathrm{O}_3 \cdot \mathrm{SiO}_2\) and \(\mathrm{Sc}_2\mathrm{O}_3 \cdot 2\mathrm{SiO}_2)\). In the Cr₂O₃—SiO₂ system,

and also in $\mathrm{TiO_2}$—$\mathrm{SiO_2}$, crystalline phases representing the product of the interaction of the metal oxide with $\mathrm{SiO_2}$ are absent. In the $\mathrm{Al_2O_3}$—$\mathrm{SiO_2}$ system there is one crystalline phase ($3\mathrm{Al_2O_3}\cdot 2\mathrm{SiO_2}$).

Thus, it has been shown that the appearance of two immiscible liquid phases in $\mathrm{Me_2O_3}$—$\mathrm{SiO_2}$ systems is caused by a disruption of the coordination ratios existing in silicate melts near liquidus temperatures.

Institute of Foundry Production
Academy of Sciences of the Ukrainian SSR

Received
5 VII 1963

CITED LITERATURE

¹ B. I. Markhasev, I. D. Sedletskii, DAN, 148, No. 4, 916 (1963).
² E. M. Levin, H. F. McMurdie, F. P. Hall, Phase Diagrams for Ceramists (1956).
³ N. A. Toropov, I. A. Bondar, Silikattechnik, 13, 4, 137 (1962).
⁴ N. A. Toropov, T. P. Kiseleva, Proc. Leningrad Technological Institute named after Lensovet, 52, 76 (1961).
⁵ S. S. Batsanov, Journal of Structural Chemistry, 3, No. 5, 616 (1962).