Reinvestigation of the Fe, Cu and Cr valences in (FeCu)Cr2S4 spinels

Abstract

The valence states of Fe, Cu, and Cr ions in single crystalline Fe1-xCuxCr2S4 have been studied using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). XPS measurements of compositions x = 0.2, 0.5, 0.6, and 0.9 indicate that the Cu and Cr ions remain mainly in the same valence state over the whole substitution range. XAS, XPS and XMCD investigations of Fe0.5Cu0.5Cr2S4 reveal a sample and preparation dependence of the Cu and Fe spectra. The comparison of the Cu 2p XAS spectrum with a literature measurement shows a change in the amount of Cu2+ explained by the appearance of an additional Cu2+ phase, possibly due to oxidation on the surface. XAS, YCPS and XMCD spectra of Fe show a Fe3+ impurity phase present in air-treated Fe0.5Cu0.5Cr2S4, especially in powdered samples. The lines related to trivalent Fe are found to disappear completely after cleaning the samples in-situ and only the lines attributed to divalent Fe remain. Since many experimental methods rely on powdered samples and measure in air or in tow vacuum, these findings provide an explanation for the differing results concerning the Fe valence obtained by these methods. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim