The crystal structure of GaPO4 at high pressure

Single-crystal structure refinements of the low-temperature quartz modification of GaPO4 were performed up to 6.80(7) GPa. The refinements were carried out in space group P3121. At ambient conditions the lattice parameters are a = 4.9043(7) Å and c = 11.4079(5) Å. They decrease to a = 4.6421(8) Å and c = 10.8022(6) Å at 6.80(7) GPa. Powder diffraction studies indicate that a pressure induced amorphization takes place at about 9 GPa. The high pressure behavior of GaPO4 is similiar to that of the homeotypic compounds quartz and GeO2, and to that of the isotypic compounds AlPO4 and AlAsO4. Structural changes with increasing pressure are characterized by an increasing c/a axial ratio, increasing distortions of the GaO4 and PO4 tetrahedra, and decreasing (Ga–O–P) angles which indicate a cooperative tilting of the tetrahedra. As in the case of other quartz-type structures, the high-pressure behavior of GaPO4 is determined by changes of the oxygen packing towards a cubic body-centered lattice. Structural changes occuring at varying temperatures are slightly different from pressure-induced changes.