PRESSURE-INDUCED TRANSFORMATIONS OF THE LOW-CRISTOBALITE PHASE OF GAPO4

Tsuneyuki et al, predicted a new high pressure phase of silica using a molecular dynamics calculation; this calculation was novel in that it predicted a new crystal structure using only a knowledge of the chemi cal composition [Nature (London) 339, 209 (1989)]. We report that the low-cristobalite phase of GaPO4 undergoes a pressure-induced transitio n at 15.9 GPa to this novel phase, which contains tetrahedral and octa hedral coordination shells of oxygen atoms around the phosphorus and g allium atoms, respectively. This pressure-induced coordination change in GaPO4 verifies a mechanism proposed for the tetrahedral-to-octahedr al coordination change in minerals that occurs in the transition zone between the upper and lower mantle.