Lanthanum doped calcium titanates: synthesis, crystal structure, thermal expansion and transport properties

Two series of perovskite-type compounds La(1-x)Ca(x)Tio(3) (x=0.2-1.0) and La2(1-x)/3CaxTiO3 (x=0, 0.1, 0.4 and 0.8) were synthesized by a ceramic technique in air (final heating 1350 degreesC). The crystal structures of the La1-xCaxTiO3 compounds were characterised at room temperature as orthorhombic in space group Pbnm (samples with 0.7 less than or equal tox less than or equal to 1) and as rhombohedral in space group R3c (x = 0.6 sample). At room temperature three different perovskite-like structures has been found for A-deficient La2(1-x)/3CaxTiO3 compound: orthorhombic Pbnm structure for the composition with x=0.8, orthorhombic Imma structure for x=0.4, and monoclinic P2/m (or possibly orthorhombic Cm2m) structure for x=0.1. The compositions were stable within the pO(2) range 21000 to 6(.)10(-16) Pa at 850-1000 degreesC. Oxygen stoichiometries and electrical conductivities of the solid solutions were investigated. At 900 degreesC within the oxygen partial pressure region 1(.)10(-15) to 0.21(.)10(5) Pa the compounds are n-type semiconductors. For La0.1Ca0.9O3 maximal conductivities were observed at 903 1000degreesC of about 50 S/cm in Ar/H-2/H2O (pH(2)O/pH(2)=0.01) and of 0.003 S/cm in air. No simple correlations between catalytic activity. electrical conductivity, and crystal structure parameters were found. (C) 2002 Elsevier Science B.V. All rights reserved.