Crystal Structures and Conduction Paths of Sc-Doped CaTiO 3 Fast Oxide-Ion Conductors with Orthorhombic Symmetry.

The crystal structures and electron density distributions (EDDs) of Ca-deficient Sc-doped CaTiO 3 fast oxide-ion conductors, Ca 0.97 (Ti 0.97 Sc 0.03 )O 3-δ (CTS3) and Ca 0.96 (Ti 0.9 Sc 0.1 )O 3-δ (CTS10), were investigated in the temperature range of 298-1173 K in N 2 to analyze the effect of composition on the perovskite structure and oxide-ion transport mechanism. CTS3 and CTS10 exhibited orthorhombic Pnma symmetry in temperature ranges of 298-1173 K and 973-1173 K, respectively, with CTS10 exhibiting lower symmetry and reduction in oxide-ion conductivity below 973 K. The EDDs of CTS3 and CTS10 at 1173 K indicated unique chemical bonds and conduction paths. CTS3 and CTS10 showed covalent bonds between (Ti,Sc) and O1 (or O2) sites. CTS3, with a lower oxide-ion conductivity than that of CTS10, exhibited pseudo-one-dimensional (1D) zig-zag curved conduction paths for oxide-ions along the a -axis, unlike previously reported curved migration paths along the b -axis in CaTiO 3 , and chemical bonds between Ca and O1 sites, indicating oxide-ion conduction suppression. In CTS10, additional conduction paths were observed along the a -axis, forming three-dimensional (3D) zig-zag curved conduction paths in the ac -plane and along the b -axis, with the weakening of the chemical bonds between the Ca and O1 sites. The oxide-ion conductivity and mobile oxide-ion concentration of CTS10 were 3.6 and 2.0 times those of CTS3, respectively, at 1173 K; the higher oxide-ion conductivity of CTS10 could be attributed to an increase in the mobile oxide-ion concentration and mobility with a 1D to 3D change in the conduction paths and a weakening of chemical bonds between the Ca and O1 sites.