Synthesis, crystal structure, and transport properties of La1−xCaxCr0.5Ti0.5O3−δ

A series of perovskite-type compounds La1−xCaxCr0.5Ti0.5O3−δ (x=0…1.0) was synthesized by a ceramic technique in air (final heating 1350 °C). The crystal structures of the compounds were characterised at room temperature for 0.0<x<0.2 as rhombohedral in space group R-3c, and for x>0.2 as orthorhombic in space group Pbnm. The lattice constants with increasing Ca contents for x>0.5 decrease noticeably and for x<0.5 remain nearly constant. All compositions were stable at pO2=21,000 to 6×10−16 Pa at 850…1000 °C. Oxygen stoichiometries and electrical conductivities of the solid solutions were investigated. The stability of the oxides against thermal dissociation evaluated by the oxygen release decreases with increasing Ca-contents. The compounds with x≥0.6 are p-type and with x<0.4 n-type semiconductors at 900 °C and oxygen partial pressures between 1×10−15 and 0.21×105 Pa. La0.5Ca0.5Cr0.5Ti0.5O3−δ and La0.6Ca0.4Cr0.5Ti0.5O3−δ change their conductivities from n-type in the range pO2<10−10 Pa and pO2<10−5 Pa, respectively, to p-type in more oxidizing atmosphere. For the compound with x=0.5 in Ar/H2/H2O gas flow (pH2O/pH2=0.53) at 900 °C a maximum conductivity of about 1 S/cm has been observed. In air the composition with x=0.75 has highest conductivity of about 40 S/cm. Partial substitution of calcium by lanthanum leads to a decrease of the average thermal expansion coefficients (TEC) from 13.1×10−6 K−1 (CaCr0.5Ti0.5O3) to 10.5×10−6 K−1 (La0.25Ca0.75Cr0.5Ti0.5O3) and 9.5×10−6 K−1 (La0.5Ca0.5Cr0.5Ti0.5O3). No simple correlations between catalytic activity, electrical conductivity, and crystal structure parameters were found.