Anomalous Thermal Expansion of HoCo0.5Cr0.5O3 Probed by X-ray Synchrotron Powder Diffraction

摘要

Mixed holmium cobaltite-chromite HoCo~(0.5)Cr~(0.5)O~(3) with orthorhombic perovskite structure (structure type GdFeO~(3), space group Pbnm ) was obtained by solid state reaction of corresponding oxides in air at 1373?K. Room- and high-temperature structural parameters were derived from high-resolution X-ray synchrotron powder diffraction data collected in situ in the temperature range of 300–1140?K. Analysis of the results obtained revealed anomalous thermal expansion of HoCo~(0.5)Cr~(0.5)O~(3), which is reflected in a sigmoidal temperature dependence of the unit cell parameters and in abnormal increase of the thermal expansion coefficients with a broad maxima near 900?K. Pronounced anomalies are also observed for interatomic distances and angles within Co/CrO~(6) octahedra, tilt angles of octahedra and atomic displacement parameters. The observed anomalies are associated with the changes of spin state of Co_(3+) ions and insulator-metal transition occurring in HoCo~(0.5)Cr~(0.5)O~(3).