Superconductivity and cobalt oxidation state in metastable Na{sub x}CoO{sub 2-{delta}}{center_dot}yH{sub 2}O (x{approx_equal}1/3; y{approx_equal}4x)

Journal Article · Sat Oct 01 00:00:00 EDT 2005 · Physical Review. B, Condensed Matter and Materials Physics

DOI:https://doi.org/10.1103/PhysRevB.72.134515· OSTI ID:20719602

Barnes, P W; Avdeev, M; Jorgensen, J D; Hinks, D G; Claus, H; Short, S ^[1]

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1. Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (United States)

We report the synthesis and superconducting properties of a metastable form of the known superconductor Na{sub x}CoO{sub 2}{center_dot}yH{sub 2}O (x{approx_equal}1/3;y{approx_equal}4x). We obtained this metastable cobaltate superconductor due to the unique way it was synthesized. Instead of using the conventional bromine-acetonitrile mixture for the Na{sup +}-deintercalation reaction, we use an aqueous bromine solution. Using this method, we oxidize the sample to a point that the sodium cobaltate becomes unstable, leading to formation of other products if not controlled. This compound has the same structure as the reported superconductor, yet it exhibits a systematic variation of the superconducting transition temperature (T{sub c}) as a function of time. Immediately after synthesis, this compound is not a superconductor, even though it contains appropriate amounts of Na{sup +} and H{sub 2}O. The samples become superconducting with low T{sub c} values after {approx}90 h. T{sub c} continually increases until it reaches a maximum value (4.5 K) after about 260 h. Then T{sub c} drops drastically, becoming nonsuperconducting approximately 100 h later. Corresponding time-dependent neutron powder diffraction data shows that the changes in superconductivity exhibited by the metastable cobaltate correspond to slow formation of oxygen vacancies in the CoO{sub 2} layers. In effect, the formation of these defects continually reduces the cobalt oxidation state causing the sample to evolve through its superconducting life cycle. Thus, the dome-shaped superconducting phase diagram is mapped as a function of cobalt oxidation state using a single sample. The width of this dome based on the formal oxidation state of cobalt is very narrow--approximately 0.1 valence units wide. Interestingly, the maximum T{sub c} in Na{sub x}CoO{sub 2}{center_dot}yH{sub 2}O occurs when the cobalt oxidation state is near +3.5. Thus, we speculate that the maximum T{sub c} occurs near the charge ordered insulating state that correlates with the average cobalt oxidation state of +3.5.

• OSTI ID: 20719602
• Journal Information: Physical Review. B, Condensed Matter and Materials Physics, Vol. 72, Issue 13; Other Information: DOI: 10.1103/PhysRevB.72.134515; (c) 2005 American Institute of Physics; Country of input: International Atomic Energy Agency (IAEA); ISSN 1098-0121
• Country of Publication: United States
• Language: English

Related Subjects

75 CONDENSED MATTER PHYSICS
SUPERCONDUCTIVITY AND SUPERFLUIDITY
ACETONITRILE
BROMINE
COBALT
COBALT COMPOUNDS
HYDRATES
LAYERS
NEUTRON DIFFRACTION
OXIDATION
OXYGEN
PHASE DIAGRAMS
SODIUM COMPOUNDS
SODIUM IONS
SUPERCONDUCTIVITY
SYNTHESIS
TIME DEPENDENCE
TRANSITION TEMPERATURE
TYPE-II SUPERCONDUCTORS
VACANCIES
VALENCE