Resistivity and thermoelectric power of Na[sub x]CoO[sub 2] (x=1.0, 0.7, and 0.6) system (original) (raw)

The role of doping and dimensionality in the superconductivity of NaxCoO2

arXiv (Cornell University), 2004

We report a complete analysis of the formal Co 3+/4+ oxidation state in Na x CoO 2 , in the interval 0.31 ≤ x ≤ 0.67. Iodometric titration and thermoelectric power confirm that a direct relationship between the Na content and the amount of Co 3+ cannot be established in this system. Creation of a significant amount of oxygen vacancies accompanies Na-ion deintercalation, keeping the formal Co valence at 3.45 + for x ≤ 0.45. To the light of new thermoelectric power data which reveals important differences between the hydrated (superconducting) and non-hydrated (non-superconducting) samples, we propose here that water plays an important "chemical" role beyond that of a spacer between the CoO 2 layers.

Role of Doping and Dimensionality in the Superconductivity of NaxCoO2

Chemistry of Materials, 2005

Dielectric Properties of Na0.7CoO2 and of the Superconducting Na0.3CoO2·1.3H2O

Journal of Superconductivity, 2005

... Rev. B 68, 18050 (2003). 11. BC Sales, R. Jim KA Affholter, P. Khalifa, GM Veith, and D. Mandrus, Phys. Rev. B 70, 174419 (2004). 12. ... 18. Yu. Feldman, N. Kozlovich, Yu. Alexandrov, R. Nigmatullin, and Ya. Ryabov, Phys. Rev. E. 54(5), 5420 (1996). 19. ...

Specific heat study of the Na0.3CoO2·1.3H2O superconductor: influence of the complex chemistry

Physica C: Superconductivity, 2004

We report results of specific heat measurements on polycrystalline samples of the layered superconductor, Na 0.3 CoO 2 ⋅1.3H 2 O. The electronic contribution to the specific heat, γ, is found to be 12.2 mJ/mol-K 2 . The feature at the superconducting transition is rather sharp, becoming broad and strongly suppressed in an applied magnetic field. The data indicate a residual normal state electronic specific heat at low temperatures, implying that there is a sizable population of normal state electrons in the samples even below T c . Inhomogeneity in the Na content, to which the superconducting state is exquisitely sensitive, appears to be the most likely explanation for these results. These results further indicate that special sample handling is required for an accurate characterization of the superconducting state in this material.

Large enhancement of the thermopower in Na$_x$CoO$_2$ at high Na doping

2006

Research on the oxide perovskites has uncovered electronic properties that are strikingly enhanced compared with those in conventional metals. Examples are the high critical temperatures of the cuprate superconductors and the colossal magnetoresistance in the manganites. The conducting layered cobaltate rmNaxCoO2\rm Na_xCoO_2rmNa_xCoO_2 displays several interesting electronic phases as xxx is varied including water-induced superconductivity and an insulating state that is

Chemical, magnetic and electronic properties of NaxCoO2 and related compounds

2012

In the last years, NaxCoO2 has experienced a renewed interest in the field of solid state science. However, NaxCoO2 is not a novel material, it has been extensively studied for decades. In the 80´s, it was investigated due to its electrochemical properties (high ionic mobility, high electrical conduction) and tested as a cathod in reversible alkaline cells, as its analogous LixCoO2. In the 90’s, its thermoelectric properties raised the interest of this material for energy harvesting at high temperature and refrigeration. However, the discovery of novel properties (i.e. high thermoelectric power or, mainly, the occurrence of superconductivity below 5K) and their possible relationship with similar phenomena found in other materials boosted a renewed interest in this highly electronic correlated system. The structural, magnetic and electronic properties of NaxCoO2 have been studied in detail in order to understand deeply some of the most fundamental aspects which drive the chemical and...

Observation of Bulk Superconductivity in NaxCoO2·yH2O and NaxCoO2·yD2O Powder and Single Crystals

Physical Review Letters, 2003

Poly-and single-crystalline NaxCoO2 has been successfully intercalated with H2O and D2O as confirmed by X-ray diffraction and thermogravimetric analysis. Resistivity, magnetic susceptibility, and specific heat measurements show bulk superconductivity with Tc close to 5 K in both cases. The substitution of deuterium for hydrogen has an effect on Tc of less than 0.2 K. Investigation of the resistivity anisotropy of NaxCoO2·yH2O single crystals shows: (a) almost zero resistivity below Tc, and (b) an abrupt upturn at T * ∼ 52 K in both the ab plane and the c direction. The implications of our results on the possible superconducting mechanism will be discussed.

Revised superconducting phase diagram of hole doped Na$_{x}$(H$_{3}$O)$_{z}$CoO$_{2}\cdot y$H$_{2}$O

Physical Review Letters, 2004

We have studied the superconducting phase diagram of NaxCoO2 ·yH2O as a function of electronic doping, characterizing our samples both in terms of Na content x and the Co valence state. Our findings are consistent with a recent report that intercalation of H3O + ions into NaxCoO2, together with water, act as an additional dopant indicating that Na sub-stochiometry alone does not control the electronic doping of these materials. We find a superconducting phase diagram where optimal TC is achieved through a Co valence range of 3.24 -3.35, while TC decreases for materials with a higher Co valence. The critical role of dimensionality in achieving superconductivity is highlighted by similarly doped non-superconducting anhydrous samples, differing from the superconducting hydrate only in inter-layer spacing. The increase of the interlayer separation between CoO2 sheets as Co valence is varied into the optimal TC region is further evidence for this criticality.

Resistivity and thermoelectric power of Na[sub x]CoO[sub 2] (x=1.0, 0.7, and 0.6) system (original) (raw)

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