Raman studies of oxide minerals: a retrospective on cristobalite phases

A brief review of early Raman studies of oxide minerals is given, emphasizing aspects yet to receive modelling. To these are added extensive unpublished infrared data on α- and β-cristobalite SiO2 and AlPO4, following a soft mode and its damping from 132 cm-1 at 20 K to about 80 cm-1 just below the AlPO4 α-β transition temperature and from 155 to 125 cm-1 in SiO2 (the transitions are of first order). Rather good data exist on the α- and β-cristobalite phases of AlPO4 and BPO4 single crystals (not powders) which are discussed in terms of the more recent controversy regarding β-cristobalite SiO2. The α-β cristobalite phase transition in AlPO4 doubles the primitive cell size on cooling, with a probable D2d to D2 point group symmetry reduction: β-AlPO4 is likely to be I4-bar 2d (D2d12) with Z = 1 formula group, and α-AlPO4 to be C 2221 (D25). For AlPO4 our results favour the model of Liu et al (1993 Phys. Rev. Lett. 70 2750) over that of Swainson and Dove (1993 Phys. Rev. Lett. 71 3610). The point group symmetries appear to be the same in cristobalite SiO2, so that the previously proposed tetragonal space group symmetry D44 (P41212) for α-SiO2 and the Oh7 (Fd3m) phase for β-SiO2 are only slow time averages that do not control selection rules or electronic band structures