Direct observation of the partial valence transition of Cu in the A-site ordered LaCu3Fe4O12-δ by soft X-ray absorption spectroscopy

Abstract

A negative-thermal-expansion-like volume contraction in LaCu3Fe4O12-δ 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 (TC, charge-transfer temperature). Although the partial valence transition of the Fe valence state was observed by Mössbauer 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-δ as a function of temperature across TC using soft x-ray absorption spectroscopy at the Cu-L3 and the OK edges. We indeed observed a sharp decrease in the valence state of Cu across the phase transition temperature TC. Nevertheless, Cu is 2.2 + and 2.7 + at 440 K and 80 K, respectively, indicates an incomplete inter-site charge transfer across TC.