Structural Study of La₂Ti₂O₇ by Convergent-Beam Electron Diffraction and Electron Microscopy

Abstract

A convergent-beam electron diffraction study has revealed that the space group of the room temperature phase of La 2 Ti 2 O 7 is P2 1 . A dark-field electron microscopy has made clear a twinned domain structure in this phase. C BED patterns obtained at above 1053 K were compatible with the space group Cmc2 1 . An incommensurate phase exists between 993 K and 1053 K. The incommensurate reflections are characterized with the vector q̄ =((1/2)-δ) ā 0 * from the fundamental reflections of the high temperature phase, a 0 * being the reciprocal lattice parameter of the high temperature phase. The value of δ increases from 0.015 to 0.017 as the temperature is raised. The transition between the room temperature phase and the incommensurate phase is of the first order, and that between the latter phase and the high temperature phase is of the second order.