High-pressure phase transformation in cotunnite-type BaCl2

Abstract

A pressure-induced phase transformation in a cotunnite-type structure compound has been observed for the first time in situ. Actually BaCl2 undergoes a reversible phase change at room temperature, which occurs at 21 GPa when increasing pressure and 15 GPa when decreasing. The high-pressure phase was indexed in a hexagonal cell. The volume change at the transition is 5% but the relative volume of the high-pressure phase in normal conditions is only 0.819. This could indicate a higher coordination number for the barium cation, still unknown in AX2 compounds. The behaviour of the low-pressure cotunnite phase appears anomalous as the bulk modulus is much higher than expected from bulk modulus-volume systematics.