Emergent charge ordering in near half doped Na${}_{0.46}$CoO${}_2$

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${}^{+3.5+\hat{I}\mu }$ and Co${}^{+3.5-\hat{I}\mu }$, $\hat{I}\mu \sim 0.06e$ 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

7 pages, 6 figures, in press Phys. Rev. B