Optical Properties of the Perovskite Solid Solution LaTiO₂N–ATiO₃(A = Sr, Ba)

Abstract

Abstract Starting from the colored oxynitride LaTiO 2 N, new perovskite‐type solid solutions have been evidenced in the systems La 1– x A x TiO 2+ x N 1– x (A = Sr, Ba) by ammonolysis at 950 °C of the corresponding oxide precursors prepared by the molten salt route. The color of the resulting powders varies progressively from the brown of LaTiO 2 N to the white of the oxide with a decrease in both the nitrogen and lanthanum contents. The optical absorption was characterized by diffuse reflectance. The nonlinear variation of the optical bandgap may result from the competition between the variation of the inductive effect between lanthanum and strontium, structural changes, and the difference in electronegativity between nitrogen and oxygen. For the compounds with a low nitrogen content the observed green shade is attributed to the presence of a nitrogen‐rich secondary phase: TiO x N y in which titanium presents a mixed‐valence state. Taking advantage of a chimie douce process, the amorphous citrate route, the highest oxidation state of titanium was stabilized until x = 0.9 in the La 0.1 Sr 0.9 Ti(O,N) 3 phase giving the powder a bright yellow color ( E g = 2.49 eV). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)