Protonic and oxide–ionic conduction in Sr m+1 (Ti 1− xIn x) mO 3 m+1− α ( m=1, 2 and ∞) at high temperature

Abstract

Indium doped Ruddlesden-Popper-type strontium titanium oxide, Sr m+1 (Ti 1− x In x ) m O 3 m+1− α ( m=1, 2 and ∞) were prepared and their conduction properties were investigated using various electrochemical methods. The structure of this series of oxides can be regarded as the alternative stacking of one rock-salt type layer (SrO) and m units of perovskite-type layers ( mSrTiO 3). In the cases of m=1, the solubility limit of indium to Ti site, x, was 0.1, and the highest conductivity appeared at x=0.07. In the cases of m=2, Sr 3(Ti 1− x In x ) 2O 7− α , the highest conductivity appeared at x=0.05. The order of ionic conductivities at 900°C was confirmed to be SrTi 0.93In 0.07O 3− α »Sr 2Ti 0.93In 0.07O 4− α >Sr 3(Ti 0.95In 0.05) 2O 7− α . Under low oxygen partial pressure like in hydrogen, all samples showed n-type electronic conduction due to the reduction of Ti 4+ to Ti 3+. The transport number of ion in hydrogen was higher in Sr 2Ti 0.93In 0.07O 4− α than in others. Dominant ionic conductive species were protons in Sr 2Ti 0.93In 0.07O 4− α and Sr 3(Ti 0.95In 0.05) 2O 7− α but oxide ions in SrTi 0.93In 0.07O 3− α . The contribution of p-type electronic conductivity under high PO 2 condition was confirmed in all samples. These results suggest that the existence of a rock-salt type layer SrO in crystal structure largely suppresses the oxide ion conductivity, while p-type electronic conductivity is not suppressed by rock-salt type layer.