Ab-initio studying of spin states of sodium cobaltate Na$_{2/3}$CoO$_2$

TL;DR

This study uses ab-initio GGA+U calculations to explore the spin states of Na$_{2/3}$CoO$_2$, revealing a stable antiferromagnetic ground state and a competition between low and intermediate spin states influenced by the Hubbard U parameter.

Contribution

It provides the first ab-initio analysis of spin states in Na$_{2/3}$CoO$_2$ using an experimentally established crystal structure, clarifying the magnetic ground state and spin state competition.

Findings

AFM-A magnetic order identified as ground state

Low spin state confirmed as most stable

Spin state competition depends on Hubbard U parameter

Abstract

Puzzling properties of sodium cobaltates Na${}_x$CoO${}_2$ become an issue of numerous recent theoretical and experimental investigations because of very high thermopower and intricate phase diagram of this material. Experiments indicated Na${}_{2/3}$CoO${}_2$ to have a kagome-like cobalt sublattice, which implies additional complexity of this strongly correlated system. In the present work we have employed the ab-initio GGA+U modeling method to investigate spin states of Na${}_{2/3}$CoO${}_2$, using an exact crystal cell established in the experiment. Stability of sodium imprint for concentration $x=2/3$ proposed in the publications has been obtained. An AFM-A type magnetic ordering has been recognized as a ground state. Moreover, a competition between low spin state (LS) and intermediate spin state (IS) has been established to depend on on-site Hubbard $U$ correction parameter. LS state has been identified as a ground state of Na${}_{2/3}$CoO${}_2$.