Superconducting transition temperature and the thickness of CoO2 planes of NaxCoO2·yH2O

The superconductivity of Na_xCoO₂·yH₂O appears in two regions of ν_Q divided by a narrow nonsuperconducting phase in the T_c–ν_Q phase diagram, where T_c and ν_Q are the superconducting transition temperature and ⁵⁹Co-nuclear quadrupole frequency, respectively. It suggests that the existence/nonexistence of the superconductivity depends on the local structure around Co sites or on the thickness of the CoO₂ planes, through the change of the crystal field. We have carried out specific heat measurements on several samples with different ν_Q values distributing over both the superconducting ν_Q regions, and found that the electronic specific heat coefficient γ does not change significantly with ν_Q. It suggests that a topological change of the Fermi surface which has been proposed as a possible origin of the existence of the two superconducting regions, does not take place with the change of the local structure around Co sites or thickness of the CoO₂ planes. We have also carried out neutron inelastic measurements on aligned crystals of Na_xCoO₂·yD₂O, and found that ferromagnetic fluctuations with two-dimensional character observed in the high temperature region lose their intensities with decreasing T and becomes inappreciable below 25 K. It indicates that the hole-pockets near the K points in the reciprocal space do not exist in these crystals. Combining these results, we can exclude, in the entire region of ν_Q, the existence of the hole-pockets, on which arguments of the possible triplet superconductivity are based. The present results are consistent with the singlet pairing which we showed in previous papers.     

Ein Natrium‐Oxocobaltat(II)‐Hydroxid: Na5[CoO3]OH ≡ Na10[CoO3]{[CoO3](OH)2}

A Sodium‐Oxocobaltate(II)‐Hydroxide: Na₅[CoO₃]OH ≡ Na₁₀[CoO₃]{[CoO₃](OH)₂} Na₁₀[CoO₃]{[CoO₃](OH)₂} has been obtained from a redox reaction between cobalt metal and CdO in the presence of NaOH and Na₂O at 600 °C (21 d) as red single crystals. The structure has been determined from single crystal data (IPDS‐data, Pnma, Z = 4, a = 988.5(1) pm, b = 1013.9(2) pm, c = 1186.3(2) pm, wR2 = 0.079). Furthermore IR data and aspects of the Madelung part of the lattice energy are presented.     

Charge compensation and oxidation in NaxCoO2−δ and LixCoO2−δ studied by XANES

A comparative study on the oxidation and charge compensation in the A_xCoO_2−δ systems, A=Na (x=0.75, 0.47, 0.36, 0.12) and Li (x=1, 0.49, 0.05), using X-ray absorption spectroscopy at O 1s and Co 2p edges is reported. Both the O 1s and Co 2p XANES results show that upon removal of alkali metal from A_xCoO_2−δ the valence of cobalt increases more in Li_xCoO_2−δ than in Na_xCoO_2−δ. In addition, the data of O 1s XANES indicate that charge compensation by oxygen is more pronounced in Na_xCoO_2−δ than in Li_xCoO_2−δ.     

Superionic Conduction in Co‐Vacant P2‐NaxCoO2 Created by Hydrogen Reductive Elimination

The layered P2‐Na_xMO₂ (M: transition metal) system has been widely recognized as electronic or mixed conductor. Here, we demonstrate that Co vacancies in P2‐Na_xCoO₂ created by hydrogen reductive elimination lead to an ionic conductivity of 0.045 S cm^?1 at 25 °C. Using in situ synchrotron X‐ray powder diffraction and Raman spectroscopy, the composition of the superionic conduction phase is evaluated to be Na_0.61(H₃O)_0.18Co_0.93O₂. Electromotive force measurements as well as molecular dynamics simulations indicate that the ion conducting species is proton rather than hydroxide ion. The fact that the Co‐stoichiometric compound Na_x(H₃O)_yCoO₂ does not exhibit any significant ionic conductivity proves that Co vacancies are essential for the occurrence of superionic conductivity.     

Electronic structure and disorder in NaxCoO2 and SrRh2O4

We discuss the electronic structure of Na_xCoO₂ from the point of view of first principles electronic structure calculations. The band structure contains low spin Co ions, with average charge 5 + x leading to a nearly full Co t_2g manifold. The bands corresponding to this manifold are narrow and separated from the O 2p bands and from the e_g bands, which are also narrow. There are two main sheets of Fermi surface, a large section derived from a_g symmetry states and small hole pockets. We find significant effects due to Na disorder on these small sections, with the result that they should be localized. This is discussed in relation to recent photoemission experiments. For comparison, we present a virtual crystal band structure of beta-SrRh₂O₄. Like Na_xCoO₂ it shows a large crystal field gap between narrow t_2g and e_g manifolds, but because of its stoichiometry is a semiconductor rather than a high carrier density metal.     

A comparative crystal chemical analysis of Ba2CoO4 and BaCoO3

Polycrystalline Ba₂CoO₄ has been synthesized by the ceramic method. X-ray and electron diffraction shows this phase to be monoclinic with a=5.8878(4); b=7.6158(6); c=10.3916(8); β=90.738(2). This compound is isostructural to Ba₂TiO₄ and the structure has been refined from X-ray powder diffraction data by using the Rietveld method. High resolution images interpreted in comparison to simulated images confirm the structure determined by X-ray diffraction. A crystal chemical study based on the cation arrays allows to establish a new relationship between Ba₂CoO₄ and 2HBaCoO₃. Moreover, the Ba subarray seems to be related to the structure of elemental Ba.     

Na2Li3CoO4, das Erste quaternäre Oxocobaltat(III) der Alkalimetalle

Na₂Li₃CoO₄, the First Quaternary Oxocobaltate(III) of the Alkali Metals For the first time we obtained Na₂Li₃CoO₄ by annealing intimate mixtures of Co₃O₄, Na₂O₂, and Li₂O [Co : Na : Li = 1 : 2.2 : 10.1; 760°C; 21 d; Ag-tube] in form of transparent red single crystals. Structure Refinement [four-circle diffractometer data; AED2; MoKα-radiation; 1016 I_o(hkl); R = 2.6%; R_w = 2.0%; space group Pnnm; Z = 4; a = 818.7(3), b = 799.4(2), c = 655.1(2) pm] confirms the isotypism to Na₂Li₃GaO₄ [2] and Na₂Li₃FeO₄ [3]. Mean Fictive Ionic Radii, MEFIR, Effective Coordination Numbers, ECoN, and the Charge Distribution were calculated. The isotypism of Na₂Li₃CoO₄ and Na₂Li₃GaO₄ is compared graphically.     

Structural difference between a superconducting sodium cobalt oxide and its related phase

Monolayer hydrate (MLH) Na_xCoO₂·y′H₂O was obtained from superconducting bilayer hydrate (BLH) Na_xCoO₂·yH₂O by partial extraction of H₂O molecules between the CoO₂ layers. Magnetization measurements indicated that electron densities in the CoO₂ layer of the MLH phase remained unchanged after the water extraction. Nevertheless, superconductivity was completely suppressed in the MLH phase. This strongly suggests that the highly 2D nature in the BLH phase due to its thick insulating layers consisting of H₂O molecules and Na⁺ ions plays an important role for inducing superconductivity.     

ChemInform Abstract: Reinvestigation of the OP4-(Li/Na)CoO2-Layered System and First Evidence of the (Li/Na/Na)CoO2 Phase with OPP9 Oxygen Stacking.

The metastable 1:1 Li/Na-stacked OP4-Li_0.42Na_0.37CoO₂ phase is synthesized from a mixture of O3-LiCoO₂ and P2-Na_0.7CoO₂ in a molar ratio of 42:58 (sealed gold tubes, 920 °C, 24 h, very fast quenching).     

Zur Kenntnis der Oxocobaltate(II) Na4[CoO3], ein Nesocobaltat(II)

On the Knowledge of Oxocobaltates(II). Na₄[CoO₃], the First Nesocobaltate(II) Newly prepared transparent, blood red single crystals of Na₄[CoO₃] (Na₂O/?CoO’? Na:Co = 4.4:1, cobalt tube, 550°C, 20d, dry argon), crystallize triclinically, P1, a = 8.14₄, b = 6.22₀, c = 5.75₈ Å; α = 117.5, β = 89.9, γ = 111.2 (diffractometer data), Z = 2. There are carbonate-like, isolated [CoO₃] groups, respectively parameters and distances see text. 2358 symmetry independent reflections (automatical four cycle diffractometer PW 1100, Mo–Kα, graphite monochromator, 4° ? Θ ? 36°), R = 0.0597. The Madelung Part of the Lattice Energy, MAPLE, Effective Coordination Numbers, ECoN and Mean Fictive Ionic Radii, MEFIR are calculated and discussed.     

Magnetic and thermoelectric properties of layered LixNayCoO2

The magnetic, thermoelectric, and structural properties of Li_xNa_yCoO₂, prepared by intercalation and deintercalation chemistry from the thermodynamically stable phase Li_0.41Na_0.31CoO₂, which has an alternating Li/Na sequence along the c-axis, are reported. For the high Li-Na content phases Li_0.41Na_0.31CoO₂ and Li_0.40Na_0.43CoO₂, a sudden increase in susceptibility is seen below 50 K, whereas for Li_0.21Na_0.14CoO₂ an antiferromagnetic-like transition is seen at 10 K, in spite of a change from dominantly antiferromagnetic to dominantly ferromagnetic interactions with decreasing alkali content. The Curie constant decreases linearly with decreasing alkali content, at the same time the temperature-independent contribution to the susceptibility increases, indicating that as the Co becomes more oxidized the electronic states become more delocalized. Consistent with this observation, the low alkali containing phases have metallic-like resistivities. The 300 K thermopowers fall between 30 μV/K (x+y=0.31) and 150 μV/K (x+y=0.83).     

Crystal structure of La0.4Sr0.6CoO2.71 investigated by TEM and XRD

The structure of the oxygen-deficient perovskite La_0.4Sr_0.6CoO_3−δ (δ=0.29) was investigated by transmission electron microscopy (TEM) and X-ray powder diffraction (XRD). Domains between 50 and 250 nm in size were observed in the electron microscope. Weak superstructure reflections were found with both X-ray and electron diffraction. Investigations of these superstructure reflections by selected area electron diffraction (SAED) and convergent beam electron diffraction (CBED) showed that the domains in a crystal are orientated in a 90° relationship. High-resolution transmission electron microscopy (HRTEM) images from the domain boundary also revealed a 90° orientation dependency. Using the symmetry of CBED patterns, the point group 4/mmm was determined. By comparing reflections from the SAED pattern with possible reflections, the space group I4/mmm (No. 139) could be isolated and finally the crystal structure was refined by Rietveld refinement.     

Die Kristallstruktur von Na4CoO4[1]

The Crystal Structure of Na₄CoO₄ According to X-Ray investigations on single crystals, Na₄CoO₄ crystallizes triclinic (P1, a = 8.64₈, b = 5.70₂, c = 6.40₀ Å, α = 123.9°, β = 98.1°, γ = 99.2°). There are almost tetrahedral CoO₄-groups; sodium is coordinated by four or five O^2?.     

Structure and properties of a novel cobaltate La0.30CoO2

The layered cobaltate La_0.30CoO₂ was prepared from Na_xCoO₂ precursor by a solid-state ionic exchange and was characterized by means of X-ray and neutron diffraction, magnetic, thermal and electric transport measurements. The compound consists of hexagonal sheets of edge-sharing CoO₆ octahedra interleaved by lanthanum monolayers. Compared to Na⁺ in the parent system, the La³⁺ ions occupy only one-third of available sites, forming a 2-dimensional superstructure. The deviation from the ideal stoichiometry La_1/3CoO₂ introduces extra hole carriers into the diamagnetic LS Co³⁺ matrix making the sample Pauli paramagnetic. The temperature dependence of the electrical conductivity in La_0.30CoO₂ follows Mott's T^−1/3 law up to about 400 K, which is in contrast with the standard metallic behavior in the Na⁺ homolog possessing the same formal doping. The experiments are complemented by electronic structure calculations for La_0.30CoO₂ and related Na_xCoO₂ systems.     

Low temperature synthesis of layered NaxCoO2 and KxCoO2 from NaOH/KOH fluxes and their ion exchange properties

We report a low temperature synthesis of layered Na_0.20Co0₂ and K_0.44CoO₂ phases from NaOH and KOH fluxes at 400°C. These layered oxides are employed to prepare hexagonal HCoO₂, Li_xCoO₂and Delafossite AgCoO₂ phases by ion exchange method. The resulting oxides were characterised by powder X-ray diffraction, X-ray photoelectron spectroscopy, SEM and EDX analysis. Final compositions of all these oxides are obtained from chemical analysis of elements present. Na_0.20Co0₂ oxide exhibits insulating to metal like behaviour, whereas AgCoO₂ is semiconducting. Dedicated to Professor C N R Rao on his 70th birthday     

Ein Natrium‐Oxocobaltat(II)‐Sulfat: Na8[CoO3][SO4]2

A Sodium Oxocobaltate(II) Sulfate: Na₈[CoO₃][SO₄]₂ Na₈[CoO₃][SO₄]₂ has been obtained from a redox reaction between cobalt metal and CdO in the presence of Na₂SO₄ and Na₂O at 550 °C (15 d) as red single crystals. The structure has been determined from single crystal data (IPDS‐data, T = 170 K, Cmcm, Z = 4, a = 806.88(9) pm, b = 2232.1(3) pm, c = 705.97(9) pm, R_all = 0.047). Magnetic properties and spectroscopic investigations are reported and discussed within the Angular‐Overlap‐Model.     

Structure and electron density analysis of electrochemically and chemically delithiated LiCoO2 single crystals

Single crystals of Li_0.68CoO₂, Li_0.48CoO₂, and Li_0.35CoO₂ were successfully synthesized for the first time by means of electrochemical and chemical delithiation processes using LiCoO₂ single crystals as a parent compound. A single-crystal X-ray diffraction study confirmed the trigonal R3¯m space group and the hexagonal lattice parameters a=2.8107(5) Å, c=14.2235(6) Å, and c/a=5.060 for Li_0.68CoO₂; a=2.8090(15) Å, c=14.3890(17) Å, and c/a=5.122 for Li_0.48CoO₂; and a=2.8070(12) Å, c=14.4359(14) Å, and c/a=5.143 for Li_0.35CoO₂. The crystal structures were refined to the conventional values R=1.99% and wR=1.88% for Li_0.68CoO₂; R=2.40% and wR=2.58% for Li_0.48CoO₂; and R=2.63% and wR=2.56% for Li_0.35CoO₂. The oxygen-oxygen contact distance in the CoO₆ octahedron was determined to be shortened by the delithiation from 2.6180(9) Å in LiCoO₂ to 2.5385(15) Å in Li_0.35CoO₂. The electron density distributions of these Li_xCoO₂ crystals were analyzed by the maximum entropy method (MEM) using the present single-crystal X-ray diffraction data at 300 K. From the results of the single-crystal MEM, strong covalent bonding was clearly visible between the Co and O atoms, while no bonding was found around the Li atoms in these compounds. The gradual decrease in the electron density at the Li site upon delithiation could be precisely analyzed.     

Ein neues Cobaltat mit Inselstruktur: Li6[CoO4]

A New Cobaltate with Isolated Anion Structure: Li₆[CoO₄] For the first time transparent, blue single crystals of Li₆[CoO₄] have been prepared (Li₂O/Na₂O/?CoO”? (Li:Na:Co = 1.3:1.3:1), Co-tube, 580°C, 22 d). Corresponding to Li₆□CoO; it is an ordered variant of the Li₂O-type of structure: P4₂/nmc; a = 653.6(1) pm, c = 465.4(1) pm; Z = 2; d_x = 2.75 g cm^?3, d_pyk = 2.71 g cm^?3 (4-circle-diffractometer-data (PW 1100), AgKα; 230 from 936 I₀(hkl); R = 9.58%, R_W = 5.25%). Parameters see text. The Madelung Part of Lattice Energy, MAPLE, and Effective Coordination Numbers, ECoN, these via Mean Fictive Ionic Radii, MEFIR, are calculated and discussed. The magnetic properties are measured in the temperature range of 14–297 K.     

Fabrication and optical conductivities of strained epitaxial NaxCoO2 thin films: x=0.5, 0.7

Epitaxial γ phase-Na_xCoO₂ thin films were deposited on (001) sapphire by the pulsed laser deposition method. To fabricate epitaxial Na_0.5CoO₂ thin films, we used a solution of iodine-dissolved acetonitrile and obtained an epitaxial Na_0.5CoO₂ thin film with a high crystallinity because of Na deintercalation of epitaxial Na_0.7CoO₂. From the spectroscopic ellipsometry analysis, we obtained the optical constants as well as the optical conductivities for the Na_0.5CoO₂ and Na_0.7CoO₂ thin films. The energy splitting between e_g and a_1g increased because of the structural strain of the Na_0.7CoO₂ thin film. It is inferred that the structural strain is the source for the lower resistivity and the preservation of the strongly correlated system up to 200 K for the Na_0.7CoO₂ thin film. On the other hand, the strain in the Na_0.5CoO₂ thin film was not affected, and the charge-ordering state and the Na content (x=0.5) only cause the charge-ordering state.     

Behavior of Co4+ ion in K2NiF4-type (Ca1+xSm1−x)CoO4

Orthorhombic K₂NiF₄-type (Ca_1+xSm_1−x)CoO₄ (0.00 ≤ x ≤0.15) with space group Bmab has been synthesized by the polymerized complex route. The cell parameters (a and b) decrease, while the cell parameter (c) increases with increasing Co⁴⁺ ion content. The global instability index (GII) indicates that the crystal stability of (Ca_1+xSm_1−x)CoO₄ is not influenced by the Co⁴⁺ ion content. (Ca_1+xSm_1−x)CoO₄ is a p-type semiconductor and exhibits hopping conductivity in the small-polaron model at low temperatures. The magnetic measurement indicates that (Ca_1+xSm_1−x)CoO₄ shows paramagnetic behavior above 5 K, and that the spin state of both the Co³⁺ and Co⁴⁺ ions is low. The Co⁴⁺ ion acts as an acceptor, and the electron transfer becomes active through the Co³⁺–O–Co⁴⁺ path as the Co⁴⁺ ions increase.