Crystallographic features of RTiO₃ (R: Rare-earth ion)

The crystallographic features of the simple perovskite oxides RTiO ₃ with R = La, Nd, Sm and Y, which are known to be correlated-electron insulators, have been examined mainly at room temperature by transmission electron microscopy. Electron diffraction patterns indicated that the oxides with R=La and Nd have the space group Pbnm. In electron diffraction patterns of SmTiO₃ and YTiO₃, on the other hand, the twin splitting of reflections due to the Pbnm structure is seen along one of the 〈100〉_c directions, where the subscript c denotes the cubic Brillouin zone. That is, it is understood that their crystal system is lowered from an orthorhombic one to a monoclinic one. The analysis of the patterns also suggested that the Jahn-Teller atomic displacement with q = 〈1/21/20〉_c should be involved in the crystal structure for R = Sm and Y, in addition to the full symmetric Jahn-Teller lattice distortion in the structure for R = La and Nd.