USE OF POLARIZED OPTICAL-ABSORPTION TO OBTAIN STRUCTURAL INFORMATION FOR NA+ ND3+ BETA''-ALUMINA/

Part of the optical absorption spectrum is calculated for the Nd3+-dop ed Na+ beta ''-alumina using a molecular-dynamics- (MD) based approach . The Judd/Ofelt (J/O) theory is modified (and several approximations removed) to treat polarized transition intensities for rare-earth ions in a solid host. Stark-level energies and eigenfunctions of the groun d state (I-4(9/2)) and excited multiplets (F-4(3/2), P-2(1/2)) are als o calculated for the local environments involved. Energies and polariz ed transition intensities between individual Stark levels are thus cal culated simultaneously for several hundreds of MD-generated environmen ts; summation gives the total bandshape. A high proportion of Beevers- Ross-site occupation (> 50%) is indicated for low Nd3+ concentrations in beta ''-alumina. The contrary is observed experimentally for high N d3+ concentrations, i.e., then midoxygen site occupation dominates. It is demonstrated that ions in different local environments contribute differently to the absorption spectrum and, particularly, how the pola rization of the various transitions changes for ions occupying differe nt sites. Temperature dependence aspects are also considered.