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Title: High-resolution neutron diffraction study of possible charge ordering in Na{sub 0.5}CoO{sub 2}

Abstract

The structure of Na{sub 0.5}CoO{sub 2}, the low-temperature insulator that separates the antiferromagnetic and normal metals in the Na{sub x}CoO{sub 2} phase diagram, is studied by high-resolution powder neutron diffraction at temperatures between 10 and 300 K. Profile analysis confirms that it has an orthorhombic symmetry structure, space group Pnmm, consisting of layers of edge-sharing CoO{sub 6} octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The oxygen content is found to be stoichiometric within 1%, indicating that the Na concentration accurately determines the electronic doping. The Na ordering creates two distinct Co sites, in parallel chains running along one crystallographic direction. The differences in their Co-O bond distances and the derived bond valence sums, reflections of the degree of charge ordering in this phase, are very small.

Authors:
;  [1]; ; ;  [2]
  1. Centre for Science at Extreme Conditions and School of Chemistry, University of Edinburgh, King's Buildings, Mayfield Road, Edinburgh, EH9 3JZ (United Kingdom)
  2. Department of Chemistry, Princeton University, Princeton, New Jersey 08544 (United States)
Publication Date:
OSTI Identifier:
20788043
Resource Type:
Journal Article
Resource Relation:
Journal Name: Physical Review. B, Condensed Matter and Materials Physics; Journal Volume: 73; Journal Issue: 13; Other Information: DOI: 10.1103/PhysRevB.73.134401; (c) 2006 The American Physical Society; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND SUPERFLUIDITY; ANTIFERROMAGNETISM; BOND LENGTHS; COBALT OXIDES; LAYERS; NEUTRON DIFFRACTION; ORTHORHOMBIC LATTICES; OXYGEN; PHASE DIAGRAMS; REFLECTION; RESOLUTION; SODIUM COMPOUNDS; SODIUM IONS; SPACE GROUPS; STOICHIOMETRY; SYMMETRY; TEMPERATURE DEPENDENCE; VALENCE

Citation Formats

Williams, A. J., Attfield, J. P., Foo, M. L., Viciu, L., and Cava, R. J.. High-resolution neutron diffraction study of possible charge ordering in Na{sub 0.5}CoO{sub 2}. United States: N. p., 2006. Web. doi:10.1103/PHYSREVB.73.1.
Williams, A. J., Attfield, J. P., Foo, M. L., Viciu, L., & Cava, R. J.. High-resolution neutron diffraction study of possible charge ordering in Na{sub 0.5}CoO{sub 2}. United States. doi:10.1103/PHYSREVB.73.1.
Williams, A. J., Attfield, J. P., Foo, M. L., Viciu, L., and Cava, R. J.. Sat . "High-resolution neutron diffraction study of possible charge ordering in Na{sub 0.5}CoO{sub 2}". United States. doi:10.1103/PHYSREVB.73.1.
@article{osti_20788043,
title = {High-resolution neutron diffraction study of possible charge ordering in Na{sub 0.5}CoO{sub 2}},
author = {Williams, A. J. and Attfield, J. P. and Foo, M. L. and Viciu, L. and Cava, R. J.},
abstractNote = {The structure of Na{sub 0.5}CoO{sub 2}, the low-temperature insulator that separates the antiferromagnetic and normal metals in the Na{sub x}CoO{sub 2} phase diagram, is studied by high-resolution powder neutron diffraction at temperatures between 10 and 300 K. Profile analysis confirms that it has an orthorhombic symmetry structure, space group Pnmm, consisting of layers of edge-sharing CoO{sub 6} octahedra in a triangular lattice, with Na ions occupying ordered positions in the interleaving planes. The oxygen content is found to be stoichiometric within 1%, indicating that the Na concentration accurately determines the electronic doping. The Na ordering creates two distinct Co sites, in parallel chains running along one crystallographic direction. The differences in their Co-O bond distances and the derived bond valence sums, reflections of the degree of charge ordering in this phase, are very small.},
doi = {10.1103/PHYSREVB.73.1},
journal = {Physical Review. B, Condensed Matter and Materials Physics},
number = 13,
volume = 73,
place = {United States},
year = {Sat Apr 01 00:00:00 EST 2006},
month = {Sat Apr 01 00:00:00 EST 2006}
}
  • Charge ordering and Na-vacancy ordering in Na{sub 0.75}CoO{sub 2} are investigated using first-principles electronic structure methods within the generalized gradient approximation (GGA) and GGA+U approximations. We find that Na ordering is determined not only by a competition between different Na site energies and Na{sup +}-Na{sup +} repulsion, but also by the Co{sup 4+}-Na{sup +} repulsion when charge localization occurs. We predict a ground-state structure for Na{sub 0.75}CoO{sub 2} in good agreement with experimental transmission electron microscopy and neutron-diffraction data and find that Co{sup 3+}/Co{sup 4+} charge ordering is strongly coupled to the Na-vacancy ordering.
  • Uranyl triacetate complexes (Cs{sub 0.5}Ba{sub 0.25})[UO{sub 2}(CH{sub 3}COO){sub 3}] (I) and Ba{sub 0.5}[UO{sub 2}(CH{sub 3}COO){sub 3}] (II) are synthesized for the first time and their structures are determined by X-ray diffraction. Both compounds crystallize in the cubic crystal system. The crystal data are as follows: a = 17.3289(7) Angstrom-Sign , V = 5203.7(4) Angstrom-Sign {sup 3}, space group I2{sub 1}3 and Z = 16 (I); a = 17.0515(8) Angstrom-Sign , V = 4957.8(4) Angstrom-Sign {sup 3}, space group I 4 bar 3d, and Z = 16 (II). In I and II, as in all uranyl triacetates studied earlier, the coordinationmore » polyhedron of the uranium atom is a hexagonal bipyramid whose vertices are occupied by the oxygen atoms of the uranyl and three acetate groups. The uranium-containing group belongs to the AB{sub 3}{sup 01} (A = UO{sub 2}{sup 2+}, B{sup 01} = CH{sub 3}COO{sup -}) crystal chemical group of uranyl complexes. It was found that compound II is isostructural to the (Rb{sub 0.50}Ba{sub 0.25})[UO{sub 2}(CH{sub 3}COO){sub 3}] studied earlier.« less
  • {sup 151}Eu and {sup 57}Fe Mossbauer spectroscopy, differential scanning calorimetry, and high-intensity high-resolution synchrotron powder diffraction were used to determine the extent of long- and short-range charge ordering below the first-order Verwey-type transition in EuBaFe{sub 2}O{sub 5}, the oxygen content of which was homogenized by annealing in sealed ampoule. The diffraction gives 0.68(5) valence unit of charge separation at 100K. Interpretation of this value as 68% of iron being long-range charge ordered correlates with the total transition entropy per formula of about 0.7 of the theoretical value 2Rln2 that would be valid for all iron atoms being fully charge ordered.more » For long- and short-range order combined, {sup 57}Fe Mossbauer spectroscopy suggests that about 90% of iron atoms occur as charge-ordered integer Fe{sup 2+} and Fe{sup 3+}. The residual 10% are the Fe{sup 2+} and Fe{sup 3+} that did not find the way to order. Local oxygen non-stoichiometry defects that revert the direction of the charge order are suggested as one of the origins of the short-range charge order. Accordingly, the long-range charge order seen by diffraction is highest in the portion of the sample that converts last upon heating, having the most ideal valence ratio.« less
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