A high-temperature Raman scattering study of the phase transitions in GaPO4 and in the AlPO4-GaPO4 system

Al1-xGaxPO4 solid solutions (x = 0.2, 0.3, 0.38, 0.7) and the pure AlPO4 (x = 0) and GaPO4 (x = 1) end members with the a-quartz-type structure were studied by Raman scattering. An investigation as a function of composition enabled the various modes to be assigned, in particular coupled and decoupled vibrations. The tetrahedral tilting modes, which have been linked to high-temperature phase transitions to alpha-quartz-type forms, were found to be decoupled. In addition, it is shown that Raman spectroscopy is a powerful technique for determining the gallium content of these solid solutions. Single crystals with x = 0.2, 0.38, and 1.0 ( GaPO4) were investigated at high temperature. The composition Al0.8Ga0.2PO4 was found to exhibit sequential transitions upon heating to the beta-quartz and beta-cristobalite forms at close to 993 K and 1073 K, respectively. Direct alpha-quartz-beta-cristobalite transitions were observed for the two other compositions at close to 1083 K and 1253 K, respectively, upon heating. The spectra of the beta-quartz and beta-cristobalite forms indicate the presence of significant disorder. Back transformation to the alpha-quartz-type form occurred readily with a hysteresis of less than 100 K for the composition x = 0.38 and for pure GaPO4. Rapid cooling was necessary to obtain the metastable alpha-cristobalite form. In contrast, for Al0.80Ga0.20PO4, the alpha-cristobalite form was obtained even upon slow cooling.