Structural Phase Transition from Rhombohedral to Cubic in LaCoO3

Abstract

Lanthanum cobalt oxide LaCoO3 has a rhombohedrally distorted perovskite structure with the crystal space group of R3c.1-3) Raccah and Goodenough have reported that the crystal space group varies with increasing temperature from R3c to R3 around K. In the high temperature phase, two kinds of inequivalent Co atoms order preferentially on alternate (111) Co 2) They have also reported a first order insulator-to-metal phase transition at 1210K accompanied with an abrupt drop of the rhombohedral angle from 60.4◦ to 60◦ keeping R3 symmetry through the transition. In addition, they reported an endothermic first order transition at 1210K, and higher order phase transitions at around 550 and 900K from the differential thermal analysis. A powder neutron diffraction study has revealed that LaCoO3 has R3c symmetry between 4.2 and 1248K, with possible exception at 668K. According to the study, the rhombohedral angle remains 60.3◦ even at 1248K. Their study does not support the first order phase transition at 1210K. In order to investigate the structural phase transition of LaCoO3, we performed X-ray powder diffraction measurement, and thermogravimetric (TG) and differential thermal analysis (DTA) at high temperatures. The results suggest a second order crystallographic phase transition from rhombohedral to cubic around 1610K, and show that any anomaly previously reported is not intrinsic one in pure LaCoO3. The powder samples of LaCoO3 for X-ray diffraction measurements were obtained from single crystal grown with a lamp-image floating zone furnace. The X-ray powder diffraction measurements were performed using a high-accuracy X-ray powder diffractometer equipped with a parabolic multilayer and parallel slit analyzer between 300 and 1700K in air atmosphere. The thermogravimetric and differential thermal analysis (TG-DTA) was performed between 300 and 1773K for both single crystalline bulk and polycrystalline powder samples with a TG-DTA (Rigaku TG-8120D) coupled with a mass spectrometer (Q-MS, Rigaku Thermo-Mass QA550