Coupled valence and spin state transition in (Pr_0.7Sm _0.3)_0.7Ca_0.3CoO₃

The coupled valence and spin state transition (VSST) taking place in (Pr_0.7Sm_0.3)_0.7Ca_0.3CoO₃ was investigated by soft x-ray absorption spectroscopy (XAS) experiments carried out at the Pr-M_4,5, Co-L_2,3, and O-1s edges. This VSST is found to be composed of a sharp Pr/Co valence and Co spin state transition centered at T^* ∼ 89.3 K, followed by a smoother Co spin-state evolution at higher temperatures. At T<T^*, we found that the praseodymium displays a mixed valence Pr3⁺/Pr4 ⁺ with about 0.13 Pr4⁺/f.u., while all the Co3⁺ is in the low-spin (LS) state. At T∼T^*, the sharp valence transition converts all the Pr4⁺ to Pr3⁺ with a corresponding Co3⁺ to Co4⁺ compensation. This is accompanied by an equally sharp spin state transition of the Co3⁺ from the low to an incoherent mixture of low and high-spin (HS) states. An involvement of the intermediate-spin (IS) state can be discarded for the Co3⁺. While above T^* and at high temperatures the system shares rather similar properties as Sr-doped LaCoO₃, at low temperatures, it behaves much more like EuCoO₃ with its highly stable LS configuration for the Co3⁺. Apparently, the mechanism responsible for the formation of Pr4⁺ at low temperatures also helps to stabilize the Co3⁺ in the LS configuration despite the presence of Co4⁺ ions. We also found out that that the Co4⁺ is in an IS state over the entire temperature range investigated in this study (10-290 K). The presence of Co3⁺ HS and Co4⁺ IS at elevated temperatures facilitates the conductivity of the material.