%0 Journal Article
%T Divacancy superstructures in thermoelectric calcium-doped sodium cobaltate
%A D. G. Porter
%A M. Roger
%A M. J. Gutmann
%A S. Uthayakumar
%A D. Prabhakaran
%A A. T. Boothroyd
%A M. S. Pandiyan
%A J. P. Goff
%J Physics
%D 2015
%I arXiv
%R 10.1103/PhysRevB.90.054101
%X 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.
%U http://arxiv.org/abs/1505.02635v1