Synthesis, crystal structure, oxygen stoichiometry, and electrical conductivity of La1−aCaaCr0.2Ti0.8O3−δ

A series of perovskite-type compounds La1−aCaaCr0.2Ti0.8O3−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-3c, 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 O2 at 20–1400 °C, and in 5% H2/Ar (pH2O/pH2=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 0.21×105 Pa the compounds with a>0.8 are p-type and with a<0.8 n-type semiconductors. La0.2Ca0.8Cr0.2Ti0.8O3−δ changes its conductivity type from n-type at pO2<10−10 Pa to p-type in more oxidizing atmosphere. For the compound with a=0.6 in 5% H2/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 [Cr3+]/[Cr4+]=1 (corresponding to an oxygen stoichiometry near 3.0), and in reducing environment to the ratio [Ti3+]/[Ti4+]=1 (corresponding to an oxygen stoichiometry near 2.95).