Unconventional charge ordering in Na0.70CoO2 below 300K?

Gavilano, J. L.; Rau, D.; Pedrini, Bill; Magishi, Ko−ichi; Weller, Martin; Hinderer, J.; Ott, H. R.; Казаков, С. М.; Karpiński, J.

doi:10.1016/j.physb.2005.01.369

Cited by 10 publications

(8 citation statements)

References 14 publications

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“…Computational studies using first principles methods [4][5][6][7][8][9][10][11][12][13][14] and low temperature experiments [15][16][17][18][19][20][21][22][23] have revealed that the fundamental physics originate from the peculiar two-dimensional layered structure of the crystal. The spin states of the edge-shared CoO 6 octahedra [20][21][22] that are responsible for high thermoelectric power, the nature of the covalent-like bonding between Co and O, 7,24 and the influence of Na ions on the electronic structure of the CoO 6 octahedra [9][10][11][12] have all been investigated. However, these factors are discussed only in terms of the Na content or the distortion of CoO 6 octahedra; to the best of our knowledge, apart from a report on the crucial role Na vacancies play in achieving low phonon thermal conductivity by our group, 25 no attention has been paid to the Na vacancies themselves, which are inevitably created when the Na content, x, is decreased from unity.…”

Section: Introductionmentioning

confidence: 99%

A First-Principles Study of the Role of Na Vacancies in the Thermoelectricity of Na x CoO2

Yoshiya

Okabayashi

Tada

et al. 2010

Journal of Elec Materi

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First principles calculations using the generalized gradient approximation to density functional theory have been carried out to evaluate formation energies of defects and the resultant changes in electronic structure of NaCoO 2 and Na 0.5 CoO 2 . The calculated formation energies confirm that Na vacancies form readily in this material, particularly through volatilization at elevated temperatures, consistent with experimentally observed behavior. Numerical analysis of the change in charge distribution upon Na vacancy formation shows that the vacancy plays a crucial role in modifying the electronic properties of the material. In these p-type thermoelectric materials, Na vacancies act as a reservoir for the minority carrier (electrons), removing them from the CoO 2 layer while simultaneously increasing the concentration of majority carriers (holes) available for conduction in the CoO 2 layer.

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Section: Introductionmentioning

confidence: 99%

A First-Principles Study of the Role of Na Vacancies in the Thermoelectricity of Na x CoO2

Yoshiya

Okabayashi

Tada

et al. 2010

Journal of Elec Materi

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“…As we outline below our data are to some extent in conflict with previously published details of structural features of the Na sublattice, in particular if x < 1 [5]. Results of diffraction experiments were discussed in relation with models describing selected static vacancy arrangements [5,9]; aspects of the motion of Na-ions have been addressed to a much lesser extent [6,11]. Our results give clear evidence that the dynamics of the Na ions is a dominating factor for the understanding of these compounds at temperatures exceeding 200 K where a growing diffusion of the Na-ions sets in.…”

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confidence: 86%

Melting of the Na Layers in SolidNa0.8CoO2

et al. 2009

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Data of23 Na NMR spectra-and relaxation measurements are interpreted as suggesting that, upon increasing temperature the Na layers in Na0.8CoO2 adopt a 2D liquid state at T = 291 K. The corresponding first order phase transition is preceded by a rapidly increasing mobility and diffusion of Na ions above 200 K. Above 291 K the 23 Na NMR response is similar to that previously observed in superionic conductors with planar Na layers.PACS numbers: 71.27.+a, 75.20.Hr, The recent activities in probing the physical behaviour of the compound series Na x CoO 2 (0.3 < x < 1) revealed a number of exciting features with respect to structural, electronic, magnetic and superconducting properties upon varying x [1,2,3,4,5,6,7,8]. Quite generally these compounds may be regarded as composed of stacks of alternating Na and Co-O layers. The role of the Na layers and the x dependence of the Na-ion arrangements have been discussed in detail [5,9]. The itinerant electronic and the magnetic degrees of freedom are concentrated in the Co-O layers [10]. Depending on the value of the Na concentration x, different ground states were identified. In this work we report a detailed 23 Na NMR investigation of the compound Na 0.8 CoO 2 . As we outline below our data are to some extent in conflict with previously published details of structural features of the Na sublattice, in particular if x < 1 [5]. Results of diffraction experiments were discussed in relation with models describing selected static vacancy arrangements [5,9]; aspects of the motion of Na-ions have been addressed to a much lesser extent [6,11]. Our results give clear evidence that the dynamics of the Na ions is a dominating factor for the understanding of these compounds at temperatures exceeding 200 K where a growing diffusion of the Na-ions sets in. Upon increasing temperature, this motion leads to a first order phase transition, most likely indicating the melting of the 2D layers of the Na sublattice at 291 K.The 23 Na nuclear magnetic resonance (NMR) experiments probed a 10.3 mg single crystalline specimen of Na x CoO 2 (x = 0.80(1)). Results of X-ray diffraction experiments confirmed the high structural quality of the sample and a quantitative chemical analysis of a specimen chosen from the same batch confirmed the Na concentration x. The crystal was characterised by magnetic susceptibility measurements using a commercial SQUID magnetometer. The NMR experiments were made in a commercial 4 He gas-flow cryostat, and the fixed external magnetic field B 0 = 7 T was provided by a commercial, ultra-shielded magnet. The 23 Na NMR spectra of the central line and the two quadrupolar satellites were separately obtained by fast Fourier transforming (FFT) the second half of the corresponding spin echo signals [18]. FIG. 1: Normalised23 Na NMR spectra of Na0.8CoO2 at different temperatures recorded in an external magnetic field of B0 = 7 T. a) Central line; b) Quadrupolar satellites measured on both sides of the central line. Their amplitudes are multiplied by a factor 5 with respect to ...

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“…It is desirable to directly probe the Na ions to confirm its dynamics. Indeed, in recent NMR studies on Na 0.7 CoO 2 powder, various anomalies in the Na NMR spectrum were seen in a similar T range [24]. ple crystallographic phases or the presence of paramagnetic volume in otherwise magnetically ordered samples [25,26,27].…”

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confidence: 93%