Charge Ordering and Lone Pair Effect in Bi_0.5Pb_0.5MnO₃

Abstract

Mn perovskite oxides with half-integer valence (3.5+) exhibit charge ordering (CO) that is the origin of intriguing properties such as the colossal magnetoresistance effect. Although CO melts below the room temperature in most of the compounds, Bi_0.5Sr_0.5MnO₃ preserves the CO state up to 475 K. It is such explained that 6s² lone pair of Bi stabilizes the charge ordering. 6s² lone pair has a steric activity and disturbs the electron transfer through the Mn-O-Mn bond. In this study, we replaced Sr by Pb with 6s² lone pair, and investigated the properties of Bi_0.5Pb_0.5MnO₃ in order to clarify the contribution of 6s² lone pair to the CO. Synchrotron XRD measurement clarified the presence of superlattice structure originating from CO and orbital ordering (OO) up to 500 K. The CO state was preserved up to 550 K, much higher than Bi_0.5Sr_0.5MnO₃. It is confirmed that increase in the fraction of ions with lone pair at the A site of Mn perovskite makes the CO state stable.