NQR and X-ray investigation of the structure of Na$_{2/3}$CoO$_{2}$ compound

TL;DR

This study combines NQR and X-ray techniques to analyze the structure and charge distribution in Na$_{2/3}$CoO$_{2}$, confirming atomic arrangements and revealing the link between sodium ordering and cobalt charge states.

Contribution

The paper provides a detailed structural and electronic characterization of Na$_{2/3}$CoO$_{2}$ using combined NQR and X-ray methods, confirming previous models and offering insights into atomic displacements and charge ordering.

Findings

X-ray diffraction confirms Na$_{2/3}$CoO$_{2}$$ structure and atomic displacements.

NQR data identifies specific cobalt and Na sites and their EFG parameters.

Charge ordering is linked to Na layer atomic arrangement.

Abstract

We have synthesized various samples of the $x=2/3$ phase of sodium cobaltate Na$_{x}$CoO$_{2}$ and performed X-ray powder diffractions spectra to compare the diffraction with the structure proposed previously from NMR/NQR experiments [H. Alloul \emph{et al.}, EPL \textbf{85}, 47006 (2009)]. Rietveld analysis of the data are found in perfect agreement with those, and confirm the concentration x=2/3 obtained in the synthesis procedure. They even give indications on the atomic displacements of Na inside the unit cell. The detailed NQR data allow us to identify the NQR transitions and electric field gradient (EFG) parameters for 4 cobalt sites and 3 Na sites. The spin-lattice and spin-spin relaxation rates are found much smaller for the non-magnetic Co$^{3+}$ sites than for the magnetic sites on which the holes are delocalized. The atomic ordering of the Na layers is therefore at the source of this ordered distribution of cobalt charges. The method used here to resolve the Na ordering and the subsequent Co charge order can be used valuably for other concentrations of Na.