Charge-orbital ordering in A-site ordered NdBaMn₂O₆ perovskite

A-site ordered perovskite-type manganites RBaMn₂O₆ (R= La, Pr, Nd, Sm, Tb) have attracted considerable attention in recent years because mutual interaction among charge, orbital, and spin degrees of freedom cause wide range of structural and physical properties. Unlike others, NdBaMn₂O₆ perovskite is unique for its multicritical feature and multiple phase transitions. So far, X-ray diffraction study determine the space-group as P4/mmm, Cmmm, and P2₁am at 450 K, 300 K, and 100 K, respectively [1]. The structure determined at low temperature (LT≈100 K) phase has a single Mn site, suggesting no charge ordering but exhibits d_x²-y² orbital ordering in the ab-plane. However, recently J. Blasco et al., report that there is no charge-orbital ordering at the LT phase [2]. In this study, we investigate the structures and electron densities to understand the charge and orbital ordering pattern at the mentioned three different temperature phases by combination of convergent-beam electron diffraction and density functional theory. We find that the LT phase has four nonequivalent Mn sites, suggesting charge ordering and the compression of octahedra indicates d_x²-y² orbital ordering.

[1] S. Yamada et al., Phys. Rev. B 95, 035101 (2017). [2] J. Blasco et al., Phys. Rev. B 103, 064105 (2021).