Physico-chemical nanomaterials science
High temperature evolution of crystal structure of Gd1-xDyxCoO3
V.M. Hreb, L.O. Vasylechko
Lviv Polytechnic National University, Lviv, Ukraine
E-mail: wasiahreb@meta.ua
Mixed gadolinium-dysprosium cobaltites Gd0.8Dy0.2CoO3 and Gd0.3Dy0.7CoO3 were prepared from Gd2O3, Dy2O3 and Co3O4 by solid-state reaction technique. Mixtures from stoichiometric amounts of oxide powders were ball-milled in ethanol for 1 h, dried and annealed in air at 1343 K for 24 h with intermediate ball-milling for 45 minutes. After cooling the products were powdered, pressed in the pellet and sintered in air at 1343 K for 96 h. 
Laboratory X-ray powder diffraction examination of both samples at room temperature confirms orthorhombic perovskite structure isotypic with GdFeO3. According to in situ high-resolution X-ray synchrotron powder diffraction investigation crystal structure remains orthorhombic (space group Pbnm) in temperature range 299–1173 K. However, strong anomalies in the lattice expansion of Gd0.8Dy0.2CoO3 and Gd0.3Dy0.7CoO3 were observed, which are reflected in a sigmoidal dependence of the unit cell dimensions and in abnormal large thermal expansion coefficients (TEC’s) with clear maxima at TEC curves at around 710 K and 733 K, respectively. Observed structural anomalies in Gd0.8Dy0.2CoO3 and Gd0.3Dy0.7CoO3 are associated with the magnetic and electronic phase transitions occurred in the DyCoO3GdCoO3 system. In particular, DyCoO3 undergoes magnetic, metal-insulator and spin-spin transitions at around 740 K, 785 K and 825 K, respectively, whereas in GdCoO3 such transitions occur at 619 K, 740 K and 650717 K [1]. According to electronic phase diagram of RCoO3 perovskites [2], DyCoO3 undergoes phase transitions from nonmagnetic dielectric to paramagnetic dielectric state at 499 K and insulator-metal transition at 753 K.
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 
Gd1-xDyxCoO3 system with increasing Dy content.
1.	Knizek K. et al. Structural anomalies associated with the electronic and spin transitions in LnCoO3 // Eur. Phys. J. B -2005.-47.-213-220.
2.	Tachibana M. et al. Evolution of electronic states in RCoO3 (R = rare earth): Heat capacity measurements // Phys. Rev. B -2008.-77.-094402.