Electron microscopy investigation of the structure of colossal magnetoresistant lanthanum manganites

Abstract

The crystal structures of calcium-doped colossal magnetoresistant lanthanum manganite, La1-x Ca xMnO3 for x=0.1 and x=0.3 were investigated using standard transmission electron microscopy techniques. The orthorhombic structure was confirmed and the structural change associated with the T~210K metal-insulator transition was investigated along with the high temperature, T~1100K structural change. The structural change at high temperature was found to correspond to an orthorhombic to cubic transition, rather than a change from an orthorhombic to rhombohedral structure. The metal-insulator transition gave rise to a measurable change in the magnitudes of the inter-atomic planar distances. In particular we found that the c axis of our x=0.3 sample changed by ~0.7%. It was found that the diffraction patterns often displayed significant numbers of superstructure reflections. This superstructure could be attributed to crystal twinning. Three types of twin were discovered during the experimental work. All three twins involved 90° rotations about the [010] and [101] crystal axes. The simplest twin discovered consists of two crystals, one rotated by 90° with respect to the other, such that the [010] axis of one crystal is parallel to the [101] axis of the other crystal. The second type of twin found comprises of two crystals arranged with their a axes perpendicular to each other, resulting in the [101] axes of each crystal being parallel. The third twin consists of three twinned crystals, and is a mixture of the first and second twins. In this third twin the [010] axis of one crystal is parallel to the [101] axis of the renaming two crystals.