Anisotropic Chemical Expansion of La_1–<i>x</i>Sr_<i>x</i>CoO_3−δ

The crystal structure and the thermal and chemical expansion of rhombohedral La_1–<i>x</i>Sr_<i>x</i>CoO_3−δ (<i>x</i> = 0.3, 0.4) perovskite were investigated by in situ high-temperature X-ray diffraction in pure oxygen and nitrogen atmospheres. The crystal structure was confirmed to change from rhombohedral (<i>R</i>3̅<i>c</i>) to cubic symmetry (<i>Pm</i>3̅<i>m</i>) with increasing temperature due to a second-order ferroelastic to paraelastic phase transition. The rhombohedral distortion at low temperature causes crystallographic anisotropy, and anisotropic thermal expansion, caused by rectification of the antiferrodistortive tilting and a minor decompression of the CoO_6/2 octahedra is reported. The onset of chemical expansion due to reduction of the oxidation state of Co was observed below the phase transition temperature, which enabled investigation of the crystallographic anisotropy of the chemical expansion. The chemical expansion was significantly larger along the <i>c</i>-axis relative to the <i>a</i>-axis, demonstrating the strong damping of the octahedral tilting with increasing oxygen vacancies induced by the thermal reduction of Co. This phenomenon caused a strong depletion of the ferroelastic phase transition temperature with decreasing partial pressure of oxygen