Samples of In-doped Ruddlesden-Popper type oxides, Srm+1(Ti1-xInx)mO3m+1, were prepared. The structures of this series were regarded as comprising the alternating stacking of one rocksalt-type layer (SrO) and m perovskite-type layers (mSrTiO3). When m was equal to unity, the solubility limit of In at Ti sites was 0.1, and the highest conductivity appeared at x = 0.07. When m was equal to 2; that is, in Sr3(Ti1-xInx)2O7, the highest conductivity appeared at x = 0.05. The order of the ionic conductivities at 900C was: SrTi0.93In0.07O3 >> Sr2Ti0.93In0.07O4 > Sr3(Ti0.95In0.05)2O7. Under low O partial pressures, such as in H, all of the samples exhibited n-type electronic conductivity; due to the reduction of Ti4+ to Ti3+. The transport number of ions under H was highest in the case of Sr2Ti0.93In0.07O4. The predominant ionic conducting species was protons in Sr2Ti0.93In0.07O4 and Sr3(Ti0.95In0.05)2O7, but oxide ions in SrTi0.93In0.07O3. It was concluded that the existence of a rocksalt-type layer, SrO, in the crystal structure largely suppressed oxide-ion conductivity, while p-type electronic conductivity was not suppressed.

Protonic and Oxide-Ionic Conduction in (Sr,Ti,In)O3 at High Temperatures. T.Shimura, K.Suzuki, H.Iwahara: Solid State Ionics, 1998, 113-115, 355-61