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arXiv · 1302.2416

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

F. Guillou·Q. Zhang·Z. Hu·C. Y. Kuo·Y. Y. Chin·H. J. Lin·C. T. Chen·A. Tanaka·L. H. Tjeng·V. Hardy
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Abstract

The coupled valence and spin state transition (VSST) taking place in (Pr0.7Sm0.3)0.7Ca0.3CoO3 was investigated by soft x-ray absorption spectroscopy (XAS) experiments carried out at the Pr-M4,5, Co-L2,3, 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 around 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.

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F. Guillou, Q. Zhang, Z. Hu, C. Y. Kuo, Y. Y. Chin, H. J. Lin, C. T. Chen, A. Tanaka, L. H. Tjeng, V. Hardy. 2013-02-11. Coupled valence and spin state transition in (Pr0.7Sm0.3)0.7Ca0.3CoO3. https://doi.org/10.1103/physrevb.87.115114

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