Iron mixed-valence compounds, BaSm(Cu0.5+xFe0.5-x)2O5+δ II:: The Fe2.5+ state and magnetic properties

Recently, a fluctuating mixed-valence state formally denoted as Fe2.5+ was observed in BaSmFe2O5+delta double-perovskite samples, having delta < 0.5 [J. Linden, P. Karen, A. Kjekshus, J. Miettinen, T. Pietari, M. Karppinetn, Phys. Rev. B 60 (1999) 15251.]. Upon cooling such samples to the Verwey-type transition temperature (T-V approximate to 200 K), a mixing-separation transition of the valence state occurs. In order to study in detail the formation of the mixed-valence state BaSm(Cu0.05+xFe0.5-x)(2)O5+delta, samples with a certain amount of Fe (0-10% and 40-50%) replaced by Cu have been characterized by Fe Mossbauer spectroscopy and magnetization measurements. All samples were found to be magnetically ordered at room temperature. The amount of Fe entering the Fe2.5+ State decreased rapidly as the Cu doping level was increased, as judged by the Mossbauer spectra. Increasing the Cu concentration also led to an increase of T-v. (C) 2000 Elsevier Science B.V. All rights reserved.