Influence of Non-Stoichiometries on the Mode of Vibration, the Amplitude of Vibration and the Infrared Intensities in beta -Alumina

The normal coordinate analysis of nonstoichiometric beta -alumina containing cations such as Na exp + , K exp + , Tl exp + , or Ag exp + with different nonstoichiometric models (Frenkel defect associated with various cation distribution in the conducting plane) is reported. Mean square amplitudes and infrared intensities have been estimated on the basis of normal mode calculations for sodium beta -alumina (stoichiometric and nonstoichiometric). The results obtained are in good agreement with experimental data. The spectral modifications that have been observed can be explained by splitting of degenerate levels and modification of selection rules due to the loss of local symmetry induced by the Frenkel defect. The good agreement between the in-plane (a, b) vibrational calculated frequencies of interstitial oxygen (O sub i ) and Raman or neutron scattering data shows that the compensating oxygen is tightly bounded to the spinel blocks. The difference between the calculated and crystallographic mean square amplitudes can be explained in terms of the static deviation of the bridging oxygen (O(5)) of 0.1 A (at 300K) from its ideal site. 29 ref.--AA