Coupling between electronic and structural degrees of freedom in the triangular lattice conductor NaxCoO2

TL;DR

This study uses neutron diffraction to explore how the crystal structure and electronic properties of NaxCoO2 are interconnected, revealing the influence of sodium ion distribution on the material's electronic phases.

Contribution

It provides detailed structural data linking sodium ion positions and octahedral shapes to electronic phases in NaxCoO2, highlighting coupling between structural and electronic degrees of freedom.

Findings

CoO6 octahedra shape depends on electron count and Na distribution

Na ion positions correlate with electronic phase diagram

Structural and electronic degrees of freedom are coupled in NaxCoO2

Abstract

The determination by powder neutron diffraction of the ambient temperature crystal structures of compounds in the NaxCoO2 family, for 0.3 < x <= 1.0, is reported. The structures consist of triangular CoO2 layers with Na ions distributed in intervening charge reservoir layers. The shapes of the CoO6 octahedra that make up the CoO2 layers are found to be critically dependent on the electron count and on the distribution of the Na ions in the intervening layers, where two types of Na sites are available. Correlation of the shapes of cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram in NaxCoO2 is made, showing how structural and electronic degrees of freedom can be coupled in electrically conducting triangular lattice systems.