Oxygen diffusion in, and thermal expansion of, SrTiO3-δ- and CaTiO3-δ-based materials

Comparative analysis of experimental data on structure, ionic conductivity, thermal expansion and oxygen nonstoichiometry of Sr0.97Ti1-xFexO3-delta and CaTi1-xFexO3-delta solid solutions shows a considerable effect of lattice ordering on their properties at temperatures below 1300 K. Increasing iron concentration in Sr-0.97(Ti,Fe)O3-delta was found to increase thermal expansion and partial ionic and electronic conductivities. Thermal expansion coefficients (TECs) of strontium titanates-ferrites increase with increasing temperature, varying from (11.7-13.8)x10(-6) K-1 at 300-720 K to (16.6-27.0)x10(-6) K-1 at 750-1100 K. In contrast, substituting titanium with iron leads to a higher ionic transport in Ca(Ti,Fe)O3-delta only for iron additions under 20 mol.% Further increase of Fe content results in formation of ordered structures and decreasing ionic conductivity. Because of stabilization of iron oxidation state and suppressing oxygen losses on heating, thermal expansion of CaTi1-xFexO3-delta ceramics is linear and approximately independent of composition. TEC values of calcium titanates-ferrites at 300-1150 K are in the narrow range (11.7-12.7)x10(-6) K-1. Oxygen permeation through CaTiO3-delta- and SrTiO3-delta-based ceramics was found to be limited by both bulk ionic conduction and surface exchange rates.