Crystal-to-stripe reordering of sodium ions in NaxCoO2 (x>=0.75)

TL;DR

This paper investigates sodium ion reordering in NaxCoO2 near room temperature, revealing phase transitions between square and striped arrangements, and a transition to a disordered stripe phase affecting electrical conduction.

Contribution

It provides a detailed understanding of sodium ion phase transitions and reordering mechanisms in NaxCoO2 at high sodium content near room temperature.

Findings

Hexagon-of-hexagons diffraction pattern at x=0.78 explained by coexisting phases

Partial melting transition to disordered stripe phase occurs below room temperature

Reordering influences electrical conduction pathways

Abstract

The sodium reordering in NaxCoO2 in the vicinity of room temperature is rationalized at high x in terms of phase transitions between square and striped phases. A striking hexagon-of-hexagons diffraction pattern observed for x=0.78 can be reproduced using coexisting square and striped phases that are related by simple shear deformations. All compositions exhibit a partial melting transition to a disordered stripe phase just below room temperature, which alters the topology of the electrical conduction pathways.