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

Starting from the colored oxynitride LaTiO2N, new perovskite-type solid solutions have been evidenced in the systems La1-xAxTiO2+xN1-x (A = Sr, Ba) by ammonolysis at 950°C of corresponding oxide precursors prepared by molten salt route. The color of the resulting powders varies progressively from the brown of LaTiO2N to the white of the oxide with the decrease of both nitrogen and lanthanum contents. The optical absorption was characterized by diffuse reflectance. The non-linear variation of the optical bandgap may result from the competition between the variation of inductive effect between lanthanum and strontium, structure changes and the difference of electronegativity between nitrogen and oxygen. For the low nitrogen contents, the observed green shade is attributed to the presence of a nitrogen-rich secondary phase: TiOxNy 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 La0.1Sr0.9Ti(O,N)3 phase giving the powder a bright yellow color (Eg = 2.49 eV).