High-resolution EELS study of the vacancy-doped metal/insulator system, Nd 1−xTiO 3, [formula omitted] to 0.33.

Abstract

High-resolution electron energy loss spectroscopy (EELS) spectra have been obtained for several members of the vacancy-doped metal/insulator perovskite system Nd 1− x TiO 3 where NdTiO 3 ( x = 0 ) is a Mott–Hubbard insulator and Nd 2/3TiO 3 ( 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 L 2,3-edge data were obtained on the title materials and the related d 0 perovskites, CaTiO 3, SrTiO 3 and BaTiO 3. The crystal structure of Nd 0.70TiO 3 ( x = 0.30 ) was refined from powder neutron data in Cmmm which is taken as a model for the structure of Nd 2/3TiO 3 ( x = 0.33 ) as well. The L-edge spectra for Nd 1− x TiO 3 show systematic changes consistent with the valence state evolution from all Ti 3+ ( x = 0.0 ) to all Ti 4+ ( x = 0.33 ). Octahedral crystal fields of 1.5 and 2.0 eV were inferred from the L-edge data for NdTiO 3 and Nd 2/3TiO 3 by comparison with calculated spectra from the literature. Broader L 3 and L 2 peak widths for Nd 2/3TiO 3 than for the other d 0 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 Nd 2/3TiO 3 is attributed to the involvement of Nd 5 d levels.