Vibrational and normal-mode analysis of stoichiometric β-Al2O3

Abstract

A complete normal-coordinate analysis of stoichiometric Na+, K+, Ag+, Tl+ β-aluminates and of Na+ β-gallates is presented. The overall agreement between calculated and experimental frequencies is excellent. Polarized Raman spectra at 300 and 16 K of stoichiometric β-aluminates, on which our assignments are based, are reported. A complete description of the normal modes is given in terms of potential energy distribution and of cartesian atomic displacements. It is shown that the in-plane and out-of-plane motions of the conducting Na+ cations are only coupled to a small extent with the spinel blocks. This is less true for heavier conducting cations. Seven spinel block modes have been found to be slightly cation dependent. A general observation is that on one hand, the largest atomic displacements are along the structural axes, on the other hand the high-frequency spectrum is dominated by the normal modes of AlO4 and Al2O7 entities, indicating some covalent character of Al-O bonds. Finally some specific modes are discussed, in particular those related to spinel-block motions as a whole, and those expected to be sensitive towards the compensating charges.