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 chemical composition [Nature (London) 339, 209 (1989)]. We report that the low-cristobalite phase of GaPO4 undergoes a pressure-induced transition at 15.9 GPa to this novel phase, which contains tetrahedral and octahedral coordination shells of oxygen atoms around the phosphorus and gallium atoms, respectively. This pressure-induced coordination change in GaPO4 verifies a mechanism proposed for the tetrahedral-to-octahedral coordination change in minerals that occurs in the transition zone between the upper and lower mantle.