Emergent charge ordering in near half doped Na$_{0.46}$CoO$_{2}$

TL;DR

This study reveals charge stripe formation and structural transitions in Na$_{0.46}$CoO$_{2}$, highlighting the interplay between Na-ion ordering, lattice distortions, and electronic states across a wide temperature range.

Contribution

First detailed neutron diffraction analysis of Na$_{0.46}$CoO$_{2}$ showing charge ordering and structural changes near half doping.

Findings

Evidence of charge stripe separation below 150K.

Na-ion dynamic transition at 300K affects lattice expansion.

Structural distortions linked to electronic state changes at 88K.

Abstract

We have utilized neutron powder diffraction to probe the crystal structure of layered Na$_{x}$CoO$_{2}$ near the half doping composition of $x=$0.46 over the temperature range of 2 to 600K. Our measurements show evidence of a dynamic transition in the motion of Na-ions at 300K which coincides with the onset of a near zero thermal expansion in the in-plane lattice constants. The effect of the Na-ordering on the CoO$_{2}$ layer is reflected in the octahedral distortion of the two crystallographically inequivalent Co-sites and is evident even at high temperatures. We find evidence of a weak charge separation into stripes of Co$^{+3.5+\epsilon}$ and Co$^{+3.5-\epsilon}$, $\epsilon\sim0.06e$ below \Tco=150K. We argue that changes in the Na(1)-O bond lengths observed at the magnetic transition at \tm=88K reflect changes in the electronic state of the CoO$_{2}$ layer