CRYSTAL-CHEMISTRY OF THE AX(2) COMPOUNDS UNDER PRESSURE

Structural phase transitions induced by high pressure in the AX(2) com pounds are reviewed. Two new structures with coordination numbers 10 a nd 11 have been added beyond the cotunnite structure. There are two pa thways between the rutile and cotunnite structures, one via the baddel eyite structure and a second via a cubic PA(3) over bar structure. The transition pathway depends on the electronic structure of the cation. No fluorite structure is induced by high pressure. A major change to the cation sublattice occurs at the transitions to the rutile structur e and to the cotunnite structure.