Transition from a phase-segregated state to single-phase incommensurate sodium ordering in gamma-NaxCoO2(x approximate to 0.53) (original) (raw)
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Phase segregation in Na x CoO2 for large Na contents
JETP Letters, 2010
We have investigated a set of sodium cobaltates (NaxCoO2) samples with various sodium content (0.67≤x≤0.75) using Nuclear Quadrupole Resonance (NQR). The four different stable phases and an intermediate one have been recognized. The NQR spectra of 59 Co allowed us to clearly differentiate the pure phase samples which could be easily distinguished from multi-phase samples. Moreover, we have found that keeping samples at room temperature in contact with humid air leads to destruction of the phase purity and loss of sodium content. The high sodium content sample evolves progressively into a mixture of the detected stable phases until it reaches the x = 2/3 composition which appears to be the most stable phase in this part of phase diagram.
^{23}Na NMR study of sodium order in Na_{x}CoO_{2} with 22 K Néel temperature
Physical Review B, 2012
We report a systematic study of the c lattice parameter in the NaxCoO2 phases versus Na content x > 0.5, in which sodium always displays ordered arrangements. This allows us to single out the first phase which exhibits an AF magnetic order at a Néel temperature TN =22 K which is found to occur for x ≈ 0.77(1). Pure samples of this phase have been studied both as aligned powders and single crystals. They exhibit identical 23 Na NMR spectra in which three sets of Na sites could be fully resolved, and are found to display T dependencies of their NMR shifts which scale with each other. This allows us to establish that the T variation of the shifts is due to the paramagnetism of the Co sites with formal charge state larger than 3 + . The existence of a sodium site with axial charge symmetry and the intensity ratio between the sets of 23 Na lines permits us to reveal that the 2D structure of the Na order corresponds to 10 Na sites on top of a 13 Co sites unit cell, that is with x = 10/13 ≈ 0.77. This structure fits with that determined from local density calculations and involves triangles of 3 Na sites located on top of Co sites (so called Na1 sites). The associated ordering of the Na vacancies is quite distinct from that found for x < 0.75.
Physical Review Letters, 2008
The sodium cobaltate family (N a x CoO 2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of coexistence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious "Curie-Weiss" metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.
Mesoscopic phase separation in NaxCoO2 (0.65⩽x⩽0.75)
Physical Review B, 2004
Nuclear magnetic resonace ͑NMR͒, EPR and magnetization measurements in Na x CoO 2 for 0.65ഛ x ഛ 0.75 are presented. While the EPR signal arises from Co 4+ magnetic moments ordering at T c Ӎ 26 K, 59 Co NMR signal originates from cobalt nuclei in metallic regions with no long range magnetic order and characterized by a generalized susceptibility typical of strongly correlated metallic systems. This phase separation in metallic and magnetic insulating regions is argued to occur below T * ͑x͒ ͑220-270 K͒. Above T * an anomalous decrease in the intensity of the EPR signal is observed and associated with the delocalization of the electrons which for T Ͻ T * were localized on Co 4+ d z 2 orbitals. It is pointed out that the in-plane antiferromagnetic coupling J Ӷ T * cannot be the driving force for the phase separation. PACS number(s): 76.60. Ϫk, 76.30.Ϫv, 71.27.ϩa
Sodium-ion diffusion and ordering in single-crystalP2-NaxCoO2
Physical Review B, 2008
Sodium-ion self-diffusion in P2͑␥͒-Na x CoO 2 has been studied using a single crystal as an electrode. Mass transport and thermodynamic properties of Na ions were measured by the potentiostatic intermittent titration technique on a single-crystal electrode with greater accuracy under well-defined diffusion geometry. Excluding the significantly slower diffusion behavior near particular Na ordered phases, average diffusion coefficients D Na are extracted to be 1.2Ϯ 0.5ϫ 10 −7 cm 2 / s for x Ͼ 0.5 and 4.1Ϯ 0.5ϫ 10 −8 cm 2 / s for x Ͻ 0.5, respectively. The existence of stable phases with special Na ordering of x ϳ 0.25, 0.33, 0.43, 0.5, 0.55, and 0.71 are verified by either the maximum or diverging signature of the x-dependent self-diffusion coefficients.
Evidence and Characterization of a SDW Transition in Na0.75CoO2 Single Crystals
The magnetic, thermal and transport properties of Na 0.75 CoO 2 single crystals grown by the floating zone (FZ) method are reported. Magnetic susceptibility, resistivity, magnetoresistance, and heat capacity data from these crystals indicate a bulk phase transition at T 1 = 22 K. These data are most consistent with the formation of an antiferromagnet spin-density-wave (SDW) at 22 K with the easy axis for magnetization nearly along the c axis. Weak and soft ferromagnetism is observed for applied magnetic fields less than 0.5 T, which suggests a slight canting of the SDW magnetization with respect to the c axis. The jump in the heat capacity at the SDW transition is 0.45 J/Kmole-Co or about 50% of the value expected from mean-field weak-coupling theory. The reduced jump and the decrease in the resistivity below T 1 are consistent with a gap for only part of the Fermi surface. The magnetoresistance is small at the SDW transition but increases in both directions reaching a value of 100% at 2 K for applied fields of 8 Tesla. The magnetoresistance data imply that the mobility of the remaining carriers is large and increases below T 1 . The observation of a SDW transition in this material is found to be sensitive to the preparation conditions and the degree of order in the Na layers. No SDW transition is observed in our polycrystalline powder with the same nominal composition (Na 0.75 CoO 2 ) and lattice constants. Differential scanning calorimetry data, however, show distinct differences between the powder and crystal, suggesting a higher degree of order in the Na layers within the crystal. The crystal exhibits a sharp phase transition at T 2 ≈ 340 K while for the powder this transition is smeared over the temperature range from 250 -310 K.
Theory of sodium ordering inNaxCoO2
Physical Review B, 2005
The ordering of Na ions in NaxCoO2 is investigated systematically by combining detailed density functional theory (DFT) studies with model calculations. Various ground state ordering patterns are identified, and they are in excellent agreement with avaliable experimental results. Our results suggest that the primary driving force for the Na ordering is the screened Coulomb interaction among Na ions. Possible effects of the Na ordering on the electronic structure of the CoO2 layer are discussed. We propose that the nonexistence of a charge ordered insulating state at x = 2/3 is due to the lack of a commensurate Na ordering pattern, whereas an extremely stable Na ordering at x = 0.5 enhances the charge ordering tendency, resulting in an insulating state as observed experimentally.
Evolution of Co charge disproportionation with Na order in Na x CoO 2
Physical Review B, 2014
59 Co NMR experiments have been performed on single crystals of the layered cobaltate NaxCoO2 with x=0.77 which is an antiferromagnet with Néel temperature TN = 22 K. In this metallic phase six Co sites are resolved in the NMR spectra, with distinct quadrupole frequencies νQ, magnetic shifts KZZ and nuclear spin lattice relaxation rates 1/T1. Contrary to the x = 1/2 or x = 2/3 phases the 3D stacking of the Na planes is not perfect for x = 0.77 but this does not influence markedly the electronic properties. We evidence that the magnetic and charge properties of the Co sites are highly correlated with each other as KZZ and (1/T1) 1/2 scale linearly with νQ. The data analysis allows us to separate the contribution ν latt Q of the ionic charges to νQ from that ν el Q due to the hole orbitals on the Co sites. We could extend coherently this analysis to all the known phases in the Na cobaltate phase diagram. The variation with x of ν latt Q is found to fit rather well numerical computations done in a point charge model. The second term ν el Q allowed us to deduce the hole concentration on the cobalts. These detailed experimental results should stimulate theoretical calculations of the electronic structure involving both the Co orbital configurations and DMFT approaches to take into account the electronic correlations.
Sodium ion and cobalt charge ordering inNaxCoO2(x∼56)
Physical Review B, 2009
We present a direct study of the Co charge order and Na ordering in three Co-layers Na x CoO 2 ͑x ϳ 5 6 ͒ single crystals. From diffuse scattering measurements, we propose a model of Na supercell which rules out the ones previously suggested in parent cobaltates with double Co layers. Via a direct measurement of the Co charge state using anomalous x-ray scattering, we also evidence a Co charge order which evolves between 200 and 290 K.