Synthesis, crystal structure, and transport properties of La 1− xCa xCr 0.5Ti 0.5O 3− δ

Abstract

A series of perovskite-type compounds La 1− x Ca x Cr 0.5Ti 0.5O 3− 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-3 c, 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 pO 2=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 0.21×10 5 Pa. La 0.5Ca 0.5Cr 0.5Ti 0.5O 3− δ and La 0.6Ca 0.4Cr 0.5Ti 0.5O 3− δ change their conductivities from n-type in the range pO 2<10 −10 Pa and pO 2<10 −5 Pa, respectively, to p-type in more oxidizing atmosphere. For the compound with x=0.5 in Ar/H 2/H 2O gas flow ( pH 2O/ pH 2=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 (CaCr 0.5Ti 0.5O 3) to 10.5×10 −6 K −1 (La 0.25Ca 0.75Cr 0.5Ti 0.5O 3) and 9.5×10 −6 K −1 (La 0.5Ca 0.5Cr 0.5Ti 0.5O 3). No simple correlations between catalytic activity, electrical conductivity, and crystal structure parameters were found.