Phase transitions under high pressure in binary Sn alloys (with In, Hg and Ga)

Crystal structures of binary alloys of Sn with In, Hg and Ga were studied under pressure with diamond-anvil cells and energy-dispersive diffraction of synchrotron radiation. The ambient-pressure simple hexagonal (sh) phase in In-Sn alloys becomes unstable under pressure above 13 GPa and decomposes into the mixture of two phases: the body-centred tetragonal (bet) phase and hexagonal close-packed (hcp) phase with compositions similar to15 and similar to25 at.% In, respectively. The hcp/bct phase boundary shifts towards Sn with increasing pressure. A similar behaviour was found for the sh phase in Hg-Sn alloys, where the hcp phase occurs at higher pressure. Another bet phase with an axial ratio similar to that in In occurs under pressure in the Hg-Sn alloy system at the composition similar to50/50. A sh phase was also found in the Ga-Sn alloy system near 80 at.% Sn at pressures above 11 GPa after heating to 150 degreesC, showing a similarity to the ambient-pressure sh phases in In-Sn and Hg-Sn alloys. Structural sequences of phases in binary alloys are discussed with respect to the valence electron concentration of the alloys, taking into account the results of the present and previous structural high-pressure studies on Sn-based and related alloys.