Phase diagram and control of thermoelectric properties of sodium cobaltate

The temperature-composition phase diagram of sodium cobaltate, NaxCoO2, was determined over the range 0.32 < x < 0.85 and 90 < T < 500K. Sodium de-intercalation was achieved using Br2/CH3CN as an oxidising agent, and the superstructures were determined using X-ray diffraction. A kaleidoscope of Na+ ion patterns were found, including both commensurate and incommensurate structures. The sodium ion patterning of some of the superstructures was solved via a model independent approach using Reverse Monte Carlo. Diffuse scattering was observed above the sodium ordering temperature. For high x, it was possible to reproduce the data using Monte Carlo simulations of short-range ordered arrangements of multi-vacancy clusters. For x = 0.5, the ordering consists of small regions of the superstructure confined to a single sodium plane. Old samples of sodium cobaltate are found to have de-intercalated so that the surface layer has a superstructure corresponding to a lower value of x than the bulk.
The thermoelectric and magnetic properties change dramatically when the superstructures change from di-vacancies to tri-vacancies. In the case of x = 0.5 we are able to understand the resonant X-ray scattering from the electronic ordering directly from the superstructure.