High-resolution EELS study of the vacancy-doped metal/insulator system, Nd1−xTiO3, x=0 to 0.33.

High-resolution electron energy loss spectroscopy (EELS) spectra have been obtained for several members of the vacancy-doped metal/insulator perovskite system Nd1−xTiO3 where NdTiO3 (x=0) is a Mott–Hubbard insulator and Nd2/3TiO3 (x=0.33), a band insulator. The insulator to correlated-metal transitions occur at x≈0.20 and x≈0.10. Both O K- and Ti L2,3-edge data were obtained on the title materials and the related d0 perovskites, CaTiO3, SrTiO3 and BaTiO3. The crystal structure of Nd0.70TiO3 (x=0.30) was refined from powder neutron data in Cmmm which is taken as a model for the structure of Nd2/3TiO3 (x=0.33) as well. The L-edge spectra for Nd1−xTiO3 show systematic changes consistent with the valence state evolution from all Ti3+ (x=0.0) to all Ti4+ (x=0.33). Octahedral crystal fields of 1.5 and 2.0eV were inferred from the L-edge data for NdTiO3 and Nd2/3TiO3 by comparison with calculated spectra from the literature. Broader L3 and L2 peak widths for Nd2/3TiO3 than for the other d0 perovskites are attributed to the more complex crystal structure of this material which includes two Nd sites with partial occupation and a more distorted Ti–O environment with low site symmetry. A markedly different O K-edge spectrum for Nd2/3TiO3 is attributed to the involvement of Nd 5d levels.