Divacancy superstructures in thermoelectric calcium-doped sodium cobaltate

TL;DR

This study investigates the superstructures in calcium-doped sodium cobaltate, revealing a stable divacancy-based superstructure that correlates with peak thermoelectric performance.

Contribution

It identifies a specific stable superstructure in Na$_x$Ca$_y$CoO$_2$ and links it to enhanced thermoelectric properties, providing new structural insights.

Findings

Stable divacancy superstructure identified

Superstructure correlates with maximum thermoelectric performance

Ca doping stabilizes superstructure across wide conditions

Abstract

We have grown single crystals of Na$_x$Ca$_y$CoO$_2$ and determined their superstructures as a function of composition using neutron and x-ray diffraction. Inclusion of Ca$^{2+}$ stabilises a single superstructure across a wide range of temperatures and concentrations. The superstructure in the Na$^+$ layers is based on arrays of divacancy clusters with Ca$^{2+}$ ions occupying the central site, and it has an ideal concentration Na$_{4/7}$Ca$_{1/7}$CoO$_2$. Previous measurements of the thermoelectric properties on this system are discussed in light of this superstructure. Na$_{4/7}$Ca$_{1/7}$CoO$_2$ corresponds to the maximum in thermoelectric performance of this system.