HIGH-PRESSURE TRANSITIONS TO A POSTCOTUNNITE PHASE IN IONIC AX(2) COMPOUNDS

The highest coordination known in ionic AX2 compounds is found in the orthorhombic cotunnite (PbCl2) structure, in which the metal ion is in ninefold coordination. Four cotunnite-structured compounds, PbCl2, BaCl2, BaBr2, and SnCl2, were investigated under high pressure by x-ray diffraction and were found to undergo phase transitions to the same postcotunnite phase at pressures between 7 and 16 GPa. The high-pressure phase is monoclinic, of space group P1121/a, Z=8, with a structure in which the metal coordination number is ten, a value which up to the present has only been observed for covalent and metallic AB2 compounds. This postcotunnite structure observed here is of considerable importance for the crystal chemistry of the AX2 compounds as under ambient conditions over 400 compounds are isostructural to PbCl2 and many others transform to a cotunnite structure at high pressures, which up to the present was the final known step in the phase-transition sequence of these materials. This new phase is 4.5±0.5% denser than the cotunnite type in the csae of PbCl2 at 16 GPa. © 1995 The American Physical Society.