Rearrangement of Sodium ordering and its effect on physical properties in $Na_xCoO_2$ system

TL;DR

This study investigates how sodium ion rearrangement in Na$_x$CoO$_2$ affects its vibrational and electronic properties, revealing structural transitions and their impact on physical behavior.

Contribution

It provides direct evidence linking Na ordering patterns to changes in physical properties and identifies a temperature-induced Na rearrangement transition.

Findings

Raman shift of A$_{1g}$ mode depends linearly on Na content

Na rearrangement occurs around x=0.5 and 240 K transition

Na ordering influences electronic transport and susceptibility

Abstract

We systematically study Raman spectroscopy of cleaved Na$_x$CoO$_2$ single crystals with 0.37 $\leq$ x $\leq$ 0.80. The Raman shift of A$_{1g}$ mode is found to be linearly dependent on Na content, while the Raman shift of E$_{1g}$ mode has an abnormal shift to high frequency around x = 0.5. The abnormal shift is ascribed to the occurrence of Na rearrangement in O1 structure. Temperature dependent Raman spectrum for x = 0.56 sample shows that Na rearrangement transition from O1 structure to H1 structure occurs around 240 K. Electronic transport and susceptibility for the sample with $x=0.56$ show a response to the Na rearrangement transition from O1 to H1 structure, and that different Na ordering pattern causes distinct physical properties. These results give a direct evidence to proved Na ordering effect on physical properties of Co-O plane.