Structural evolution of La₂Ti₂O₇ at elevated temperatures

Structural evolution of a La₂Ti₂O₇ ferroelectric compound possessing perovskite-type slabs at elevated temperatures was investigated using the single-crystal X-ray diffraction technique. The monoclinic low-temperature phase (L) transformed into the orthorhombic high-temperature phase (H) via an incommensurately modulated phase (IC) between ∼989 and ∼1080 K. The L–IC transition was considered to be of the first order, with the L+IC two-phase co-existing region between ∼989 and ∼1027 K. The structure of IC was determined from the (3+1)-dimensional superspace representation with a modulation vector q = αa_o (α ≃ 0.49), where a_o is the a-axis vector of the basic cell. The structural modulation originated from the variation of the tilt angle of the TiO₆ octahedra in the perovskite-type slab in association with small positional displacements of La atoms. The IC–H transition took place at ∼1080 K and was close to the second order. During the IC–H transition, nanoscale flat plate domains having either a cell twin of the L-type structural modules or a cell twin of the alternating H- and L-type structural modules began to appear in the approximant structure of IC. The thickness of the flat plate domains then grew rapidly along the modulation vector in proportion to (½ − α)⁻¹ as α approached ½ with decreasing temperature. In the two-phase L+IC co-existing region, the IC phase consisting of the two types of cell twins was gradually replaced with the low-temperature monoclinic phase L, which is not cell twinned but rather twinned macroscopically by the L-type structural modules.