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

Abstract Recently, a fluctuating mixed-valence state formally denoted as Fe 2.5+ was observed in BaSmFe 2 O 5+ δ double-perovskite samples, having δ T V ≈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(Cu 0.5+ x Fe 0.5− x ) 2 O 5+ δ , samples with a certain amount of Fe (0–10% and 40–50%) replaced by Cu have been characterized by 57 Fe Mossbauer spectroscopy and magnetization measurements. All samples were found to be magnetically ordered at room temperature. The amount of Fe entering the Fe 2.5+ 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 .