Physico-chemical nanomaterials science
Anomalous thermal behaviour of crystal structure EuCo0.5Cr0.5O3 probed by X-ray synchrotron powder diffraction
V.M. Hreb, L.O. Vasylechko
Lviv Polytechnic National University, Lviv, Ukrain
E-mail: wasiahreb@meta.ua
The rare earth (R) cobaltites and chromites with perovskite-like structure are used as electrode materials for SOFC, thermoelectrics, catalysts and sensory materials. Properties of RCoO3 materials strongly depend on spin state of Co3+ ions in octahedral sites. 
New mixed europium cobaltite-chromite EuCo0.5Cr0.5O3 was prepared from constituent oxides Eu2O3, Co3O4 and Cr2O3 by solid state reaction in air at 1573 K. X-ray powder diffraction examination revealed orthorhombic perovskite structure isotypic with GdFeO3. The unit cell dimensions of EuCo0.5Cr0.5O3 are in good agreement with the structural data of the parent EuCoO3 and EuCrO3 compounds, thus proving formation of the continuous solid solution in the EuCoO3EuCrO3 system. According to the in situ high-resolution X-ray synchrotron powder diffraction study EuCo0.5Cr0.5O3 remains orthorhombic in a broad temperature range of 298–1173 K. No symmetry related structural changes were observed. However, an anomalous behaviour of the crystal parameters was observed, which is reflected in a sigmoidal temperature dependence of the unit cell parameters and in increase of the linear thermal expansion coefficients with broad maxima near 900 K. Evidently, similar to the “pure” rare earth cobaltites, these observed anomalies are caused by an excitation of Co3+ ions from low spin to the higher spin states and the coupled magnetic and insulator-metal (I-M) transition. According to the electronic phase diagram of the RCoO3 perovskites [1], EuCoO3 undergoes phase transitions from nonmagnetic dielectric to paramagnetic dielectric state at ~350 K or ~400 K (structural data or magnetic susceptibility measurement), and I-M transition at ~620 K or ~680 K (heat capacity and resistivity measurement). As it was expected, the observed anomalies in the mixed cobaltite-chromite EuCo0.5Cr0.5O3 are less pronounced than in the “pure” EuCoO3. Extra structural anomalies were detected in the temperature dependence of the selected bond lengths and octahedra tilt angles. Similar to previous investigation of RCoO3 series the increase of cooperative rotations of corner-shared CoO6 octahedra with temperature leads to reduction of the bandwidth of Co(3d)–O(2p) interactions and increase of the spin-spin and I-M transition temperature in EuCo1-xCrxO3 system.
1.	Tachibana M. et al. Evolution of electronic states in RCoO3 (R = rare earth): Heat capacity measurements // Phys. Rev. B -2008.-77.-094402.