Commensurate structural modulation in the charge- and orbitally-ordered phase of the quadruple perovskite (NaMn$_3$)Mn$_4$O$_{12}$

TL;DR

This study reveals a commensurate structural modulation in (NaMn₃)Mn₄O₁₂ associated with charge and orbital order transition at 176 K, confirmed by diffraction techniques, aligning with theoretical models of charge ordering.

Contribution

It provides the first detailed structural analysis of charge and orbital order in a quadruple perovskite, demonstrating a commensurate modulation linked to the charge order transition.

Findings

Structural modulation with q=(1/2,0,-1/2) observed below T_CO

Charge and orbital order consistent with Goodenough-Kanamori model

Large entropy change at T_CO indicating a significant phase transition

Abstract

By means of synchrotron x-ray and electron diffraction, we studied the structural changes at the charge order transition $T_{CO}$=176 K in the mixed-valence quadruple perovskite (NaMn$_3$)Mn$_4$O$_{12}$. Below $T_{CO}$ we find satellite peaks indicating a commensurate structural modulation with the same propagation vector q =(1/2,0,-1/2) of the CE magnetic order that appears at low temperature, similarly to the case of simple perovskites like La$_{0.5}$Ca$_{0.5}$MnO$_3$. In the present case, the modulated structure together with the observation of a large entropy change at $T_{CO}$ gives evidence of a rare case of full Mn$^{3+}$/Mn$^{4+}$ charge and orbital order consistent with the Goodenough-Kanamori model.