Solid state synthesis, structure and transport properties of compositions in the CaRu1−xTixO3−δ system

Double B perovskite-type CaRu1−xTixO3−δ series with x = 0, 0.1, 0.2…0.9, 1.0 were synthesized in air by solid state reactions from simple ruthenium and titanium oxides and CaCO3. Crystal structures of CaRu1−xTixO3 samples have been refined in space group Pnma (GdFeO3-type of structure) assuming statistical distribution of Ru and Ti species over the octahedral sites. Electrical conductivity and thermal expansion were investigated at t = 20–1000 °C and pO2 = 1–21000 Pa. Apart from CaRuO3 with a combined p-/n-type conductivity depending on temperature and pO2, all other compositions are n-type semiconductors. The conductivity of CaRuO3 decreases with increasing temperature (metal-type). The Ti-containing compositions with x = 0.1–0.2 change their conductivity dependence from metal-type at relatively low oxygen partial pressures (pO2) to semi-conducting-type at relatively high ones. The materials with x > 0.2 have the semi-conducting-type dependence at all investigated pO2. Disproportionation of Ru4+ into Ru3+ and Ru5+ cations is considered for explanation of the conductivity dependences on temperature and pO2. The investigated compositions show high catalytic activity for propene oxidation and no activity for NO2 reduction.