1 Nature 2003 Vol: 424(6948):527-529. DOI: 10.1038/nature01877

Superconductivity phase diagram of NaxCoO2·1.3H2O

The microscopic origin of superconductivity in the high-transition-temperature (high-Tc) copper oxides remains the subject of active inquiry; several of their electronic characteristics are well established as universal to all the known materials, forming the experimental foundation that all theories must address. The most fundamental of those characteristics, for both the copper oxides and other superconductors, is the dependence of the superconducting Tc on the degree of electronic band filling. The recent report of superconductivity1 near 4 K in the layered sodium cobalt oxyhydrate, Na0.35CoO21.3H2O, is of interest owing to both its triangular cobalt–oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors. Here we show that the superconducting Tc of this compound displays the same kind of behaviour on chemical doping that is observed in the high-Tc copper oxides. Specifically, the optimal superconducting Tc occurs in a narrow range of sodium concentrations (and therefore electron concentrations) and decreases for both underdoped and overdoped materials, as observed in the phase diagram of the copper oxide superconductors. The analogy is not perfect, however, suggesting that NaxCoO21.3H2O, with its triangular lattice geometry and special magnetic characteristics, may provide insights into systems where coupled charge and spin dynamics play an essential role in leading to superconductivity.

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Figures
Figure 1: Powder X-ray diffraction patterns (Cu K radiation) for NaxCoO2·yH2O samples prepared using different concentrations of the bromine de-intercalant.Inset, an enlargement of the 006 reflections for each sample, highlighting the shift in the layer spacing as a function of sodium content. The NaxCoO2yH2O samples were prepared by chemically de-intercalating sodium from Na0.7CoO2 using bromine as an oxidizing agent. Na0.7CoO2 (0.5 g) was stirred in 20 ml of a Br2 solution in acetonitrile at room temperature for five days. Bromine concentrations representing substoichiometric (0.5 ), stoichiometric (1 ), and molar excess (10–100 ) relative to sodium content were employed. ('1 ' indicates that the amount of Br2 used is exactly the amount that would theoretically be needed to remove all of the sodium from Na0.7CoO2.) The product was washed several times with acetonitrile and then water, and then dried briefly under ambient conditions. The sodium content of the phases was determined by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) method. Very high Na diffusion coefficients facilitate homogenization of the Na contents of the samples at ambient temperature. Figure 2: Zero field cooled d.c. magnetization for superconducting samples of NaxCoO2·1.3H2O.Data were obtained for x = 0.29, 0.30 and 0.32, using a Quantum Design PPMS magnetometer, Hdc = 5 Oe). Inset, loss in weight of single phase NaxCoO21.3H2O (x = 0.26 and 0.32) samples heated extremely slowly in O2 (0.25 degrees per minute) illustrating the method by which we distinguish the amount of crystal water (the higher-temperature weight loss) from the intergrain water (the lower-temperature weight loss). The change in weight that occurs on loss of crystal water is seen to be essentially the same in both low-Na and high-Na content materials. Figure 3: Zero-field cooled a.c. magnetization for all superconducting NaxCoO2·yH2O samples. (Hdc = 3 Oe, Hac = 5 Oe, f = 10 kHz) Magnetization data for the weakly superconducting samples x = 0.45 and 0.40 are shown in the inset. Figure 4: The superconducting phase diagram for NaxCoO2·1.3H2O.Main panel, Tc as a function of x as determined from the a.c. susceptibility measurements in . Inset, schematic representation of the layered crystal structure of NaxCoO21.3H2O. Triangular layers of CoO6 edge-shared octahedra are shown in a polyhedral representation.
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References
  1. Takada, K. et al. Superconductivity in two-dimensional CoO2 layers. Nature 422, 53-55 , (2003) .
    • . . . The recent report of superconductivity1 near 4 K in the layered sodium cobalt oxyhydrate, Na0.35CoO21.3H2O, is of interest owing to both its triangular cobalt–oxygen lattice and its generally analogous chemical and structural relationships to the copper oxide superconductors . . .
    • . . . Like the high-Tc superconductors, the NaxCoO21.3H2O crystal structure1 consists of electronically active planes (in this case, edge-sharing CoO6 octahedra) separated by layers (in this case, Nax1.3H2O) that act as spacers, to yield electronic two-dimensionality, and also act as charge reservoirs (see below) . . .
  2. Foo, M. L. et al. Chemical instability of the cobalt oxyhydrate superconductor under ambient conditions. Solid State Commun 127, 33-37 , (2003) .
    • . . . Thermogravimetric analysis of all the samples on very slow heating in oxygen (Fig. 2 inset) showed that their behaviour was identical to that reported previously for Na0.3CoO2yH2O (ref. 2) . . .
    • . . . Observations that the lower hydrates with closer CoO2–CoO2 interplanar distances are not superconducting above 2 K (ref. 2), and that Tc decreases under pressure12, indicate that the two-dimensional character of the structure is important . . .
  3. Foussassier, C., Matjeka, G., Reau, J.-M. & Hagenmuller, P. Sur de noveaux bronzes oxygenes de formulae NaxCoO2 (x 1). Le system cobalt-oxygene-sodium. J. Solid State Chem. 6, 532-537 , (1973) .
    • . . . This increase in layer separation with decreasing sodium content for the hydrated superconducting phase is similar to that observed in the dehydrated NaxCoO2 phase3, 4 . . .
  4. Baskaran, G. An electronic model for CoO2 layer based systems: Chiral RVB metal and superconductivity. Preprint at Link , (2003) .
    • . . . This increase in layer separation with decreasing sodium content for the hydrated superconducting phase is similar to that observed in the dehydrated NaxCoO2 phase3, 4 . . .
    • . . . Preliminary correlation of the chemical doping due to the Na content and the true electronic doping state of the CoO2 planes can be accomplished by electron counting in the context of electronic pictures already being developed for both the dehydrated NaxCoO2 and NaxCoO21.3H2O phases (see, for example, refs 4–9) . . .
  5. Kumar, B. & Shastry, B. S. Superconductivity in CoO2 layers and the resonating valence bond mean field theory of the triangular lattice t-J model. Preprint at Link , (2003) .
    • . . . However, it has been pointed out that the trigonal distortion of the CoO6 octahedra in these structures will probably lead to splitting of the t2g band5, 10 . . .
  6. Singh, D. J. Electronic structure of NaCo2O4. Phys. Rev. B 61, 13397-13402 , (2000) .
    • . . . Electronic structure calculations for Na0.5CoO2 (ref. 6) indicate that the t2g band would be completely filled at x = 1, and that an 'ordinary' semiconductor is expected . . .
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    • . . . However, it has been pointed out that the trigonal distortion of the CoO6 octahedra in these structures will probably lead to splitting of the t2g band5, 10 . . .
    • . . . Observations of unusual electrical transport properties in the host material Na0.7CoO2 itself suggest that coupled spin and charge dynamics may be implicated in the superconductivity, although there are significant differences from copper oxide systems10, 11 . . .
  11. Wang, Y., Rogado, N. S., Cava, R. J. & Ong, N. P. Spin entropy as the likely source of enhanced thermopower in NaxCo2O4. Nature 423, 425-428 , (2003) .
    • . . . Observations of unusual electrical transport properties in the host material Na0.7CoO2 itself suggest that coupled spin and charge dynamics may be implicated in the superconductivity, although there are significant differences from copper oxide systems10, 11 . . .
  12. Lorenz, B., Cmaidalka, J., Meng, R. L. & Chu, C. W. Effect of hydrostatic pressure on the superconductivity in NaxCoO2yH2O. Preprint at Link , (2003) .
    • . . . Observations that the lower hydrates with closer CoO2–CoO2 interplanar distances are not superconducting above 2 K (ref. 2), and that Tc decreases under pressure12, indicate that the two-dimensional character of the structure is important . . .
  13. Anderson, P. W. Resonating valence bonds: A new type of insulator. Mater. Res. Bull. 8, 153-160 , (1973) .
    • . . . Though the intrinsically complex materials chemistry of the NaxCoO21.3H2O superconductor makes it difficult to characterize, we believe that potentially fruitful comparisons to the copper oxides, and the fact that this compound may represent the literal embodiment of Anderson's original proposal for the resonating valence bond state13, make it worthy of further study. . . .
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