Jahn-Teller distortions and charge, orbital and magnetic orders in NaMn7O12

TL;DR

This study uses band structure calculations to resolve inconsistencies in the crystal and magnetic structures of NaMn7O12, predicting a new low-temperature structure with distinct Jahn-Teller distortions and charge/orbital orders.

Contribution

The paper provides a revised crystal structure for NaMn7O12 and clarifies the nature of its magnetic, charge, and orbital orders, correcting previous experimental interpretations.

Findings

New low-temperature crystal structure predicted

AFM-CE magnetic order stabilizes elongated Mn3+ octahedra

Orbital correlations exist in the cubic phase

Abstract

With the use of the band structure calculations we demonstrate that previously reported [Nat. Materials {\bf 3}, 48 (2004)] experimental crystal and magnetic structures for NaMn$_7$O$_{12}$ are inconsistent with each other. The optimization of the crystal lattice allows us to predict a new crystal structure for the low temperature phase, which is qualitatively different from the one presented before. The AFM-CE type of the magnetic order stabilizes the structure with the elongated, not compressed Mn$^{3+}_B$O$_6$ octahedra, striking NaMn$_7$O$_{12}$ out of the list of the anomalous Jahn-Teller systems. The orbital correlations were shown to exist even in the cubic phase, while the charge order appears only in the low temperature distorted phase.