Raman studies of oxide minerals: a retrospective on cristobalite phases

Abstract

A brief review of early Raman studies of oxide minerals is given, emphasizing aspects yet toreceive modelling. To these are added extensive unpublished infrared data onα- andβ-cristobalite SiO2 and AlPO4, following a soft mode and its damping from132 cm−1 at 20 Kto about 80 cm−1 just below the AlPO4 α–β transition temperatureand from 155 to 125 cm−1 in SiO2 (the transitions are of first order). Rather good data exist on theα- andβ-cristobalitephases of AlPO4 and BPO4 single crystals (not powders) which are discussed in terms of the more recent controversy regardingβ-cristobaliteSiO2. Theα–β cristobalite phasetransition in AlPO4 doubles the primitive cell size on cooling, with a probableD2d toD2 point groupsymmetry reduction: β-AlPO4 is likely to be (D2d12) withZ = 1 formulagroup, and α-AlPO4 tobe C 2221 (D25).For AlPO4 our results favour the model of Liu et al (1993 Phys. Rev. Lett. 70 2750) over that of Swainson andDove (1993 Phys. Rev. Lett. 71 3610). The point group symmetries appear to be the same in cristobaliteSiO2, so that the previously proposed tetragonal space group symmetryD44 (P41212)for α-SiO2 andthe Oh7 (Fd3m) phasefor β-SiO2 are only slow time averages that do not control selection rules or electronic band structures.