Transformation of three-connected silicon in BaSi2

Solid-solid transitions in trimorphic BaSi₂ have been investigated up to 40 kbar and 1000°C by X-ray powder technique in quenched samples. All transformations between orthorhombic BaSi₂I with isolated Si-tetrahedra, trigonal BaSi₂II with corrugated Si-layers, and cubic BaSi₂III with a three-dimensional three-connected Si-net can be performed in approximately 5 min at high-pressure-high-temperature conditions. At ambient conditions the difference in molar volume between BaSI₂I and BaSi₂III is relatively large (ΔV_I–III = ?6.79 cm³/mole) and that between BaSi₂III and BaSi₂II very small (ΔV_III-II = ?0.05 cm³/mole). Consequently in the pressure-temperature phase diagram the boundary (I–III) shows a strong pressure dependence contrary to that of (III-II) which is less dependent on variation of pressure. The triple point between the three solid phases is near 11 kbar and 925°C. Substitution of divalent metal and quadrivalent metalloid can easily influence the phase relations in BaSi₂.