Synchrotron radiation topography studies of the phase transition in LaGaO₃ crystals

An investigation of the orthorhombic to rhombohedral phase transformation occurring at 145°C in lanthanum gallate has been conducted using white beam synchrotron X-ray topography (WBSXRT). The existence of the first order transition was confirmed by differential thermal analysis and X-ray diffractometer powder analysis. Subsequent to this, synchrotron white beam Laue patterns were recorded in situ as a function of temperature, during the transition. Before the transition point was reached, (112)_orth type reflection twinning was found to be dominant although a small amount of (110)_orth type twinning was also observed in the same crystal. Beyond the transition point, not only did the structural change become evident but also reflection twinning on the (110)_rhom planes was observed. The scale of this twinning became finer as the temperature was increased beyond the transition temperature. The twinning observed in both the low and high temperature phases gives rise to deformation of the (011)_rhom surface plane which creates problems for the potential use of this material as a substrate for growing high T_c superconducting epitaxial layers.