Coupled valence and spin state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3

Abstract

The coupled valence and spin state transition (VSST) taking place in (Pr${}_{0.7}$Sm${}_{0.3}$)${}_{0.7}$Ca${}_{0.3}$CoO${}_{3}$ 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 ${T}^{*}$ \ensuremath{\sim} 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 Pr${}^{3+}$/Pr${}^{4+}$ with about 0.13 Pr${}^{4+}$/f.u., while all the Co${}^{3+}$ is in the low-spin (LS) state. At $T\ensuremath{\sim}{T}^{*}$, the sharp valence transition converts all the Pr${}^{4+}$ to Pr${}^{3+}$ with a corresponding Co${}^{3+}$ to Co${}^{4+}$ compensation. This is accompanied by an equally sharp spin state transition of the Co${}^{3+}$ 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 Co${}^{3+}$. While above ${T}^{*}$ and at high temperatures the system shares rather similar properties as Sr-doped LaCoO${}_{3}$, at low temperatures, it behaves much more like EuCoO${}_{3}$ with its highly stable LS configuration for the Co${}^{3+}$. Apparently, the mechanism responsible for the formation of Pr${}^{4+}$ at low temperatures also helps to stabilize the Co${}^{3+}$ in the LS configuration despite the presence of Co${}^{4+}$ ions. We also found out that that the Co${}^{4+}$ is in an IS state over the entire temperature range investigated in this study (10--290 K). The presence of Co${}^{3+}$ HS and Co${}^{4+}$ IS at elevated temperatures facilitates the conductivity of the material.