Divacancy superstructures in thermoelectric calcium-doped sodium cobaltate
Abstract
We have grown single crystals of NaxCayCoO2 and determined their superstructures as a function of composition using neutron and x-ray diffraction. Inclusion of Ca2+ 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 Ca2+ ions occupying the central site, and it has an ideal concentration Na4/7Ca1/7CoO2. Previous measurements of the thermoelectric properties on this system are discussed in light of this superstructure. Na4/7Ca1/7CoO2 corresponds to the maximum in thermoelectric performance of this system.
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