Control of Electrons through Patterning of Superstructures in Sodium Cobaltate

Sodium Cobaltate (NaxCoO2) has emerged as a material of exceptional scientific and technological interest since it is among the best P-type thermoelectric materials. The superstructures in pure NaxCoO2 templates the Coulomb landscape on the Co layers and is found to control the physical properties. The combination of the high electrical conductivity in the Co layers with the low thermal conductivity due to the rattling of sodium ions in cages, are the precise conditions for thermoelectric materials with high figures of merit. Replacing Na by divalent ions was reported to lead to a dramatic improvement in thermoelectric performance. The superstructures of NaxCayCoO2 and NaxSryCoO2 have been determined by Laue diffraction using neutrons on SXD at ISIS and x-rays at Royal Holloway. Reverse Monte-Carlo methods were used to determine Na ion patterning including the locations of the divalent ions. Co and O displacements were also determined that show buckling following the Na structure. In the doped systems we find completely new multi-vacancy clusters. Di-vacancies form in Ca doped systems where the divalent ion sits at the central site. In the Sr doped systems two new superlattices are observed, explained by the clustering of separated Sr ions with associated vacancies. Multiple valence states have been detected by NMR, which is a local probe, but spatial charge ordering in the cobalt layer has not previously been observed. We report new measurements using Resonant X-ray Scattering on the Materials and Magnetism beamline I16 at Diamond. We find resonant x-ray scattering with the same periodicity as the sodium superstructure, directly demonstrating that the electronic ordering in these cobalt layers is controlled by the sodium ordering. We are able to reproduce the energy, polarisation and azimuthal dependencies of the resonant x-ray scattering in calculations using the FDMNES code