Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La 1− aCa aCr 0.2Ti 0.8O 3− δ

Abstract

A series of perovskite-type compounds La 1− a Ca a Cr 0.2Ti 0.8O 3− δ ( a=0.6–1.0) was synthesized by a ceramic technique in air (final heating 1350 °C). For 0.4< a<0.6 the crystal structures of the compounds cooled in air were characterised at room temperature as rhombohedral in space group R-3 c, and for a>0.6 as orthorhombic in space group Pbnm. The lattice constants for a>0.8 decrease noticeably with increasing Ca content and for a<0.8 they remain nearly constant. All compositions were stable in air and in Ar/1 Pa O 2 at 20–1400 °C, and in 5% H 2/Ar ( pH 2O/ pH 2=0.01) at 850–1000 °C. Oxygen stoichiometries and electrical conductivities of the solid solutions with a=0.6 to 1.0 were investigated. Increasing Ca-contents decrease the stability of the oxides in respect to the thermal dissociation with oxygen release. At 900 °C within the oxygen partial pressure region 1×10 −15 to 0.21×10 5 Pa the compounds with a>0.8 are p-type and with a<0.8 n-type semiconductors. La 0.2Ca 0.8Cr 0.2Ti 0.8O 3− δ changes its conductivity type from n-type at pO 2<10 −10 Pa to p-type in more oxidizing atmosphere. For the compound with a=0.6 in 5% H 2/Ar flow at 1000 °C, a maximum conductivity of about 50 S/cm has been observed. In air the composition with a=0.95 has the highest conductivity of about 1 S/cm. In an oxidizing environment the maximal conductivities correspond to the ratio [Cr 3+]/[Cr 4+]=1 (corresponding to an oxygen stoichiometry near 3.0), and in reducing environment to the ratio [Ti 3+]/[Ti 4+]=1 (corresponding to an oxygen stoichiometry near 2.95).