Possible helical-spin/charge ordering in perovskite-type oxide Ba_1-xLa_xFeO₃

Perovskite-type iron oxides with anomalously high valence have attracted wide attentions due to the versatile magnetic properties. Cubic perovskite SrFeO₃ with Fe⁴⁺ exhibits a variety of topological spiral magnetic ordering [1]. Additionally, (Sr, Ba)_2/3La_1/3FeO₃ with Fe^3.67+ exhibits a spin/charge ordering (SCO) phase where the magnetic ordering and charge ordering are coupled with each other [2]. However, iron perovskite oxides with regions where topological spiral magnetic ordering and charge ordering have not been reported. In this study, we focused on helimagnetic BaFeO₃ [3], which exhibits insulating behavior in contrast to SrFeO₃, and Ba_1-xLa_xFeO₃ was synthesized systematically, in order to discover novel helical-spin/charge ordering phases.

Ba_1-xLa_xFeO_3-δ was synthesized by solid state reaction method, followed by sintering with oxidizing agent under high pressure. A series of perovskite Ba_1-xLa_xFeO₃ (x = 0 ~ 0.5) were obtained and found to remain cubic in all x. A complete phase diagram was established to reveal the evolution from helimagnetic order to antiferromagnetic order through two types of SCO phases by La doping. Interestingly, a presumably helimagnetic transition with first-order nature was observed for Ba_0.8La_0.2FeO₃, Additionally, charge disproportionation of Fe⁴⁺ was observed by Mössbauer spectroscopy measurements. Based on above results, we propose a plausible novel SCO phase in x = 0.2 with an incommensurate spiral structure and charge disproportionation.