Synchrotron X-ray topography studies of twinning and the phase transition at 145°C in LaGaO₃ single crystals

An investigation of (i) twinning in the room temperature orthorhombic phase, (ii) the orthorhombic-to-rhombohedral phase transformation occurring at 145°C and (iii) twinning in the high temperature rhombohedral phase, all occurring in lanthanum gallate single crystals has been undertaken using white-beam synchrotron X-ray topography (WBSXRT). At room temperature, WBSXRT results obtained from the (110)_orth surface, orthorhombic crystals indicated the presence of "mirror" twinning on (112)_orth, (112)_orth, (110)_orth)_orth and (112)_orth planes. Differenttial thermal analysis confirmed the existence of a first-order phase transformation at 145°C. WBSXRT observations of the associated structural change (orthorhombic to rhombohedral) correlated well with the results of temperature-dependent powder diffraction results which are presented in the companion paper. WBSXRT also revealed the detailed spatial characteristics of the transition on the microstructural scale, as well as the nature of twinning in the rhombohedral phase, with the latter occurring on (110)_rhomb planes. In all three cases, results were consistent with the transmission electron microscopy results presented in the companion paper. The influence of both the twinning and the phase transition on the potential use of LaGaO₃ single crystals as substrates for high T_c superconductor epilayers is discussed.