Low temperature phase transitions and crystal structure of Na0.5CoO2

TL;DR

This study investigates the crystal structure and phase transitions of Na0.5CoO2, revealing layered CoO6 octahedra, ordered Na ion chains, and temperature-induced structural and magnetic changes.

Contribution

It provides detailed structural characterization and links phase transitions at 87 K and 53 K to entropy changes and possible structural components.

Findings

Na0.5CoO2 has layered CoO6 octahedra with ordered Na chains.

Transitions at 87 K and 53 K involve entropy changes.

Structural component suggested for the 87 K transition.

Abstract

The crystal structure of Na0.5CoO2, determined by powder neutron diffraction, is reported. The structure consists of layers of edge-shared CoO6 octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The Na ions form one-dimensional zigzag chains. Two types of Co ions, which differ only subtly in their coordination by oxygen, are also found in chains. Specific heat measurements show that the transitions observed at 87 K and 53 K in the resistivity and magnetic susceptibility are accompanied by changes in entropy. Electron diffraction studies suggest that the 87K transition may have a structural component.