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Patterning of sodium ions and the control of electrons in sodium cobaltate


Sodium cobaltate (NaxCoO2) has emerged as a material of exceptional scientific interest due to the potential for thermoelectric applications1,2, and because the strong interplay between the magnetic and superconducting properties has led to close comparisons with the physics of the superconducting copper oxides3. The density x of the sodium in the intercalation layers can be altered electrochemically, directly changing the number of conduction electrons on the triangular Co layers4. Recent electron diffraction measurements reveal a kaleidoscope of Na+ ion patterns as a function of concentration5. Here we use single-crystal neutron diffraction supported by numerical simulations to determine the long-range three-dimensional superstructures of these ions. We show that the sodium ordering and its associated distortion field are governed by pure electrostatics, and that the organizational principle is the stabilization of charge droplets that order long range at some simple fractional fillings. Our results provide a good starting point to understand the electronic properties in terms of a Hubbard hamiltonian6 that takes into account the electrostatic potential from the Na superstructures. The resulting depth of 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. Thus we conclude that the Na+ ion patterning has a decisive role in the transport and magnetic properties.

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Figure 1: Na x CoO 2 has Na + ions intercalated between CoO 2 layers.
Figure 2: Neutron diffraction showing bulk three-dimensional ordering of Na+.
Figure 3: Real space superstructure.
Figure 4: Coulomb potential in the Co plane calculated using ordered superstructures.


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We thank S. Lee, J. Irvine, P. Radaelli, A. Daoud-Aladine, K. Kiefer, D. Argyriou, R. Coldea and M. Nohara for discussions, and T. Bowcock and A. Washbrook for the use of the MAP2 supercomputer at the University of Liverpool.

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Correspondence to M. Roger.

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Supplementary Figures

This file contains Supplementary Figures 1-6 with Legends. The figures show the determination of the commensurate supercell, the scattering from other multi-vacancy cluster models, a sodium re-ordering transition, the high-temperature phase, the relation to the electrical properties, and Monte Carlo simulations as a function of composition. (PDF 579 kb)

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Roger, M., Morris, D., Tennant, D. et al. Patterning of sodium ions and the control of electrons in sodium cobaltate. Nature 445, 631–634 (2007).

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