Anomalous Thermal Expansion of HoCo(0.5)Cr(0.5)O(3) Probed by X-ray Synchrotron Powder Diffraction

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).