Patterning of sodium ions and the control of electrons in sodium cobaltate
NaxCoO2 is a promising candidate for thermoelectric applications to cool electronic devices and convert wasted heat into electricity. We have used neutrons to better understand its properties. Neutron diffraction from NaxCoO2 reveals a kaleidoscope of sodium ordering patterns as a function of concentration. Electrostatic forces stabilize sodium vacancies into clusters (shown in red in the figure) and these order long range at simple fractional fillings. The sodium superstructures strongly affect the electronic behaviour of NaxCoO2. They cause some of the charge carriers in the Co layers to be trapped, and in this way the sodium ordering can be used to control the electronic properties. The Coulomb landscape in the figure shows how the mobile charge carriers are confined to restricted regions of the plane, enhancing their interactions and explaining their anomalous properties. The vacancy clusters form cages in which sodium ions can rattle, disrupting phonon propagation without affecting the electrical conductivity and resulting in NaxCoO2's thermoelectric properties.
Roger M, Morris DJP, Tennant DA, Gutmann MJ, Goff JP, Hoffmann J-U, Feyerherm R, Dudzik E, Prabhakaran D, Boothroyd AT, Shannon N, Lake B and Deen PP
Nature 445, 631 (2007). http://dx.doi.org/10.1038/nature05531