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.