The AIPO4 polymorphs structure in the light of Raman and IR spectroscopy studies

Raman spectra of low-temperature polymorphic forms of AlPO4 (berlinite, phosphocristobalite and phosphotridymite) are presented. The interpretation of the spectra has been carried out using the model of [PO4](3-) tetrahedron isolated by Al3+ cations. The theoretical number of Raman active modes has been determined using the factor group analysis. Raman and IR spectra have been compared regarding adequate selection rules. The crystalline field splitting and Davydov effects have been discussed. It has been shown that AlPO4 polymorphs, usually classified as a framework structure, because of the similarity to SiO2, should be rather treated as an orthophosphate structure. Thus the model of the oxygen bridge, commonly used for interpretation of framework structures, should not be used in the case of AlPO4. (C) 2000 Elsevier Science B.V. All rights reserved.