Interplay between Magnetism and Na concentration in $Na_xCoO_2$

TL;DR

This study uses density functional calculations to explore how varying sodium concentration affects the structural, magnetic, and electronic properties of Na_xCoO_2, revealing specific magnetic tendencies and Na ion arrangements.

Contribution

The paper provides detailed computational insights into the relationship between Na concentration and magnetic behavior in Na_xCoO_2, aligning with experimental findings.

Findings

Na ions shift within the basal plane at lower x to reduce electrostatic repulsion.

Strong A-type antiferromagnetism observed at x=0.75.

Other Na-deficient systems tend to be weakly ferromagnetic.

Abstract

Through comprehensive density functional calculations, the crystallographic, magnetic and electronic properties of $Na_xCoO_2$ ($x$ = 1, 0.875, 0.75, 0.625 and 0.50) were investigated. We found that all Na ions in $NaCoO_2$ and $Na_{0.875}CoO_2$ share the basal coordinates with O ions. However, as $x$ decreases, some of Na ions move within the basal plane in order to reduce the in-plane Na$-$Na electrostatic repulsion. Magnetically, there was strong tendency for type A antiferromagnetism in the $Na_{0.75}CoO_2$ system, while all other Na deficient systems had a weaker ferromagnetic tendency. The results on magnetism were in excellent agreement with the experiments.