A high-temperature Raman scattering study of the phase transitions in GaPO(4) and in the AlPO(4)-GaPO(4) system.

Al(1-x)Ga(x)PO(4) solid solutions (x = 0.2, 0.3, 0.38, 0.7) and the pure AlPO(4) (x = 0) and GaPO(4) (x = 1) end members with the α-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 β-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 (GaPO(4)) were investigated at high temperature. The composition Al(0.8)Ga(0.2)PO(4) was found to exhibit sequential transitions upon heating to the β-quartz and β-cristobalite forms at close to 993 K and 1073 K, respectively. Direct α-quartz-β-cristobalite transitions were observed for the two other compositions at close to 1083 K and 1253 K, respectively, upon heating. The spectra of the β-quartz and β-cristobalite forms indicate the presence of significant disorder. Back transformation to the α-quartz-type form occurred readily with a hysteresis of less than 100 K for the composition x = 0.38 and for pure GaPO(4). Rapid cooling was necessary to obtain the metastable α-cristobalite form. In contrast, for Al(0.80)Ga(0.20)PO(4), the α-cristobalite form was obtained even upon slow cooling.