Experimental and first-principles calculation study of the pressure-induced transitions to a metastable phase in GaPO4 and in the solid solution AlPO4−GaPO4

alpha-Quartz-type gallium phosphate and representative compositions in the AlPO4-GaPO4 solid solution were studied by x-ray powder diffraction and absorption spectroscopy, Raman scattering, and by first-principles calculations up to pressures of close to 30 GPa. A phase transition to a metastable orthorhombic high-pressure phase along with some of the stable orthorhombic Cmcm CrVO4-type material is found to occur beginning at 9 GPa at 320 degrees C in GaPO4. In the case of the AlPO4-GaPO4 solid solution at room temperature, only the metastable orthorhombic phase was obtained above 10 GPa. The possible crystal structures of the high-pressure forms of GaPO4 were predicted from first-principles calculations and the evolutionary algorithm USPEX. A predicted orthorhombic structure with a Pmn2(1) space group with the gallium in sixfold and phosphorus in fourfold coordination was found to be in the best agreement with the combined experimental data from x-ray diffraction and absorption and Raman spectroscopy. This method is found to very powerful to better understand competition between different phase transition pathways at high pressure.