Divacancy superstructures in thermoelectric calcium-doped sodium cobaltate

We have grown single crystals of NaxCayCoO2 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 CoO2. Previous measurements of the thermoelectric properties on this system are discussed in light of this superstructure. Na 4/7 Ca 1/7 CoO2 corresponds to the maximum in thermoelectric performance of this system.