title: A high-temperature Raman scattering study of the phase transitions in GaPO(4) and in the AlPO(4)-GaPO(4) system.
abstract: 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.
authors.affiliation: Laboratoire des Colloïdes, des Verres et des Nanomatériaux, UMR CNRS 5587, Université Montpellier II, cc026, Place E Bataillon, F-34095 Montpellier Cedex 5, France. Laboratoire de Physicochimie de la Matière Condensée, UMR CNRS 5617, Université Montpellier II, cc003, Place E Bataillon, F-34095 Montpellier Cedex 5, France.
authors.forename: E
authors.initials: E
authors.lastname: Angot
authors.forename: R Le
authors.initials: RL
authors.lastname: Parc
authors.forename: C
authors.initials: C
authors.lastname: Levelut
authors.forename: M
authors.initials: M
authors.lastname: Beaurain
authors.forename: P
authors.initials: P
authors.lastname: Armand
authors.forename: O
authors.initials: O
authors.lastname: Cambon
authors.forename: J
authors.initials: J
authors.lastname: Haines