Sodium Ordering and the Control of Properties in Sodium Cobaltate

• Main Author: Pryce Morris, David Jonathan
• Published: University of Liverpool 2007
• Subjects: 546.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.486940

Sodium Cobaltate has emerged as a material of exceptional scientific interest, but the role of sodium ordering has until now been poorly understood. The main result of this thesis is the determination of the Na superstructures using neutron diffraction. The organisational principles and the effects on the physical properties are also determined. Single crystals grown using the floating-zone technique were screened and cleaved to a single grain using SXD at ISIS. This instrument surveys huge regions of reciprocal space using the time-of-flight neutron Laue diffraction technique, and the 3D data sets reveal a kaleidoscope of superlattice diffraction patterns, including 12-fold rings and a hexagon-of-hexagons. The ordering and its associated distortion field are governed by pure electrostatics, and the organisational principle is the stabilisation of charge droplets that order long range at some simple fractional fillings. The whole data set can be understood in terms of a square lattice of trivacancy clusters that transform to stripe phases via shear distortions. The results provide a good starting point to understand the electronic properties in terms of a Hubbard Hamiltonian that takes into account the electrostatic potential for the sodium superstructure. The resulting depth of the potential wells in the Co layer is greater than the single-particle hopping kinetic energy and as a consequence, holes preferentially occupy the lowest potential regions. The multivacancy clusters form cages in which Na ions vibrate, disrupting the propagation of phonon excitations, and leading to excellent thermoelectric properties.