X-ray absorption spectroscopy on layered cobaltates Na_xCoO_2

TL;DR

This study uses soft x-ray absorption spectroscopy to investigate local symmetry deviations and electronic structure in layered cobaltates Na_xCoO_2, revealing doping-dependent crystal field splitting and site-specific symmetry deviations.

Contribution

It provides new insights into the local symmetry and electronic structure of Na_xCoO_2 through polarization-dependent measurements and cluster calculations.

Findings

Deviation of local trigonal symmetry is temperature independent between 25 K and 372 K.

Different symmetry deviations observed for Co^{3+} and Co^{4+} sites.

Doping-dependent energy splitting (10Dq) between t_{2g} and e_g levels identified.

Abstract

Measurements of polarization and temperature dependent soft x-ray absorption have been performed on Na_xCoO_2 single crystals with x=0.4 and x=0.6. They show a deviation of the local trigonal symmetry of the CoO_6 octahedra, which is temperature independent in a temperature range between 25 K and 372 K. This deviation was found to be different for Co^{3+} and Co^{4+} sites. With the help of a cluster calculation we are able to interpret the Co L_{23}-edge absorption spectrum and find a doping dependent energy splitting between the t_{2g} and the e_g levels (10Dq) in Na_xCoO_2.