On the high-pressure phase transition in GaPO

X-ray diffraction (XRD) experiments have been carried out on quartz-like $\alpha -$GaPO4 at high pressure and room temperature. A transition to a high pressure disordered crystalline form occurs at 13.5 GPa. Slight heating using a YAG infrared laser was applied at 17 GPa in order to crystallize the phase in its stability field. The structure of this phase is orthorhombic with space group Cmcm. The cell parameters at the pressure of transition are a=7.306 Å, b=5.887 Å and c=5.124 Å