Doping induced spin state transition in Li$_x$CoO$_2$ as studied by the GGA+DMFT calculations

TL;DR

This study uses GGA+DMFT calculations to explore how doping levels in Li$_x$CoO$_2$ influence the spin states of cobalt ions, revealing a transition from high to low spin states driven by crystal field effects.

Contribution

It demonstrates the role of Hund's exchange and crystal field splitting in spin state transitions in Li$_x$CoO$_2$, providing new insights into its magnetic properties.

Findings

High spin state at low doping levels

Transition to low spin state with increased delithiation

Co$^{3+}$ remains in low spin state across all doping levels

Abstract

Magnetic properties of Li$_x$CoO$_2$ for $x = 0.94, 0.75, 0.66$ and $0.51$ were investigated in frames of method combining Generalized Gradient Approximation with Dynamical Mean--Field Theory (GGA+DMFT). We found that a delicate interplay between Hund's exchange energy and $t_{2g}-e_g$ crystal field splitting is responsible for the high spin to low spin state transition for Co$^{4+}$ ions. The GGA+DMFT calculations show that at small doping level the Co$^{4+}$ ions adopt high spin state, while delithiation results in increase of the crystal field splitting and low spin state becomes preferable. The Co$^{3+}$ ions were found to stays in the low spin configuration for any $x$.