The crystal structure of the quartz-type form of GaPO4 and its temperature dependence

The structure analyses of the quartz-type form of GaPO4 crystals were carried out by using X-ray diffraction intensities at six points of temperatures from 293 K to 747 K. Temperature dependence of the thermal and positional parameters of atoms was examined by applying the conventional harmonic structure-factor expression and also a generalized structure factor expression. Thermal vibrations of oxygen atoms are highly anisotropic, being contributed by dominant librational motions of Ga–O–P planes around the Ga–P lines like those of the corresponding moiety in quartz or berlinite. Both the atomic positions and the directions of the principal axes of the thermal ellipsoids are quite reluctant to change with varying temperature unlike those of quartz or berlinite. According to the anharmonic refinements, the Ga–O and P–O bond distances are about 1.84 Å and 1.54 Å, respectively, which are longer than the values obtained in the harmonic refinements. In GaPO4, the Ga–O and P–O bonds are rigid, but O–Ga–O and O–P–O angles are rather flexible, and the structure with the smaller unit-cell than that of the berlinite form of AlPO4 is maintained by bending the Ga–O–P bonds to such a small angle as 133°. It is indicated that, if GaPO4 took the high symmetry configuration of the high temperature form of berlinite, the tetrahedral angles would be too strongly distorted.