Crystal structures and phase transition in (Sr_0.8Ce _0.2)(Mn_1-yCo_y)O₃ (y = 0 and 0.2): The influence of Jahn-Teller distortion

We have studied the crystal structures of (Sr_0.8Ce _0.2)(Mn_1-yCo_y)O₃ (y = 0 and 0.2) using neutron diffraction. Both (Sr_0.8Ce_0.2)MnO ₃ and (Sr_0.8Ce_0.2)(Mn_0.8Co _0.2)O₃ have a tetragonal structure in space group I4/mcm at room temperature, and the octahedral tilt angle around the c-axis is nearly the same. The only significant difference is the shape of the Mn(Co)O ₆ octahedron: it is elongated in (Sr_0.8Ce _0.2)MnO₃ due to the cooperative Jahn-Teller (JT) effect, but essentially regular in (Sr_0.8Ce_0.2)(Mn _0.8Co_0.2)O₃ due to the absence of JT-active Mn³⁺ ions. With increasing temperature, both compounds undergo a continuous phase transition at around 400 °C to a cubic structure in , with no indication of a distinct transition in (Sr_0.8Ce _0.2)MnO₃ from the removal of the static JT distortion. In addition, the temperature dependence of the octahedral tilt angle is very similar in the two samples, implying that the JT distortion has minimal effect on the octahedral tilting and the phase transition to cubic. X-ray absorption near-edge structure (XANES) analysis indicates that the Ce oxidation state is predominantly 4+ in both samples. The electrical conductivity is higher in (Sr_0.8Ce_0.2)MnO₃ than in (Sr _0.8Ce_0.2)(Mn_0.8Co_0.2)O₃ in the temperature range studied (100-900 °C).