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Abstract:
A negative-thermal-expansion-like volume contraction in LaCu3Fe4O12-delta was proposed to result from an inter-site charge transfer between Fe and Cu, namely from Cu3+-Fe3+ at room temperature to Cu2+-Fe3.75+ above 393 K(T-C, charge-transfer temperature). Although the partial valence transition of the Fe valence state was observed by Mossbauer spectroscopy and the BVS method, there is no direct observation of the electron transfer to the Cu site in this material until now. In this work, we have studied the valence state of Cu ions in LaCu3Fe4O12-delta as a function of temperature across T-C using soft x-ray absorption spectroscopy at the Cu-L-3 and the O-K edges. We indeed observed a sharp decrease in the valence state of Cu across the phase transition temperature T-C. Nevertheless, Cu is 2.2 + and 2.7 + at 440 K and 80 K, respectively, indicates an incomplete inter-site charge transfer across T-C.
