The high pressure behaviour of the cotunnite and post-cotunnite phases of PbCl2 and SnCl2

The high pressure behaviour of PbCl2 and SnCl2 has been investigated by angular-dispersive X-ray diffraction. Under ambient conditions, these two compounds adopt the cotunnite structure (Pnam, Z = 4) for which the cation has the highest coordination number found among the ionic AX(2) compounds. A crystallographic transition was observed at 16 GPa for both compounds. The structure of the high pressure phase is a distorted orthorhombic Co2Si structure: it is monoclinic (P112(1)/a, Z = 8) and the coordination number has increased to ten. The mechanism of the transition occurs in two minimal steps: Pnam --> P2(1)/m --> P2(1)/a. The pressure dependence of the cell constants, volume and the equation-of-state parameters are given. In the cotunnite phase, B-0 and B-0' are 34 GPa and 7.4, and 31 GPa and 4.9, for PbCl2 and SnCl2, respectively. The bulk moduli of both compounds triple above the transition. The same monoclinic structure accounts for the high pressure phases of all the cotunnite-structured compounds investigated up to now. This structure type was previously unknown for AX(2) compounds and is now the final step for the high pressure sequence of the phase transitions in these compounds. It is probable that the AO(2) dioxides adopt this structure in the megabar range.