Orbital frustration at the origin of the magnetic behavior in LiNiO2

We report on the ESR, magnetization and magnetic susceptibility measurements performed over a large temperature range, from 1.5 to 750 K, on high-quality stoichiometric LiNiO2. We find that this compound displays two distinct temperature regions where its magnetic behavior is anomalous. With the help of a statistical model based on the Kugel'-Khomskii Hamiltonian, we show that below T_of ~ 400 K, an orbitally-frustrated state characteristic of the triangular lattice is established. This then gives a solution to the long-standing controversial problem of the magnetic behavior in LiNiO2.Comment: 5 pages, 5 figures, RevTex, accepted in PR

Localized versus itinerant magnetic moments in Na0.72CoO2

Based on experimental 59Co-NMR data in the temperature range between 0.1 and 300 K, we address the problem of the character of the Co 3d-electron based magnetism in Na0.7CoO2. Temperature dependent 59Co-NMR spectra reveal different Co environments below 300 K and their differentiation increases with decreasing temperature. We show that the 23Na- and 59Co-NMR data may consistently be interpreted by assuming that below room temperature the Co 3d-electrons are itinerant. Their magnetic interaction appears to favor an antiferromagnetic coupling, and we identify a substantial orbital contribution corb to the d-electron susceptibility. At low temperatures corb seems to acquire some temperature dependence, suggesting an increasing influence of spin-orbit coupling. The temperature dependence of the spin-lattice relaxation rate T1-1(T) confirms significant variations in the dynamics of this electronic subsystem between 200 and 300K, as previously suggested. Below 200 K, Na0.7CoO2 may be viewed as a weak antiferromagnet with TN below 1 K but this scenario still leaves a number of open questions.Comment: 8.7 pages, 6 Figures, submitted to Phys. Rev.

Coupling between electronic and structural degrees of freedom in the triangular lattice conductor NaxCoO2

The determination by powder neutron diffraction of the ambient temperature crystal structures of compounds in the NaxCoO2 family, for 0.3 < x <= 1.0, is reported. The structures consist of triangular CoO2 layers with Na ions distributed in intervening charge reservoir layers. The shapes of the CoO6 octahedra that make up the CoO2 layers are found to be critically dependent on the electron count and on the distribution of the Na ions in the intervening layers, where two types of Na sites are available. Correlation of the shapes of cobalt-oxygen octahedra, the Na ion positions, and the electronic phase diagram in NaxCoO2 is made, showing how structural and electronic degrees of freedom can be coupled in electrically conducting triangular lattice systems.Comment: 15 pages, 1 tables, 6 figures Submitted to Physical Review

Searching for Stable Na-ordered Phases in Single Crystal Samples of gamma-NaxCoO2

We report on the preparation and characterization of single crystal gamma phase NaxCoO2 with 0.25 < x < 0.84 using a non-aqueous electrochemical chronoamperemetry technique. By carefully mapping the overpotential versus x (for x < 0.84), we find six distinct stable phases with Na levels corresponding to x ~ 0.75, 0.71, 0.50, 0.43, 0.33 and 0.25. The composition with x ~0.55 appears to have a critical Na concentration which separates samples with different magnetic behavior as well as different Na ion diffusion mechanisms. Chemical analysis of an aged crystal reveals different Na ion diffusion mechanisms above and below x_c ~ 0.53, where the diffusion process above x_c has a diffusion coefficient about five times larger than that below x_c. The series of crystals were studied with X-ray diffraction, susceptibility, and transport measurements. The crystal with x = 0.5 shows a weak ferromagnetic transition below T=27 K in addition to the usual transitions at T = 51 K and 88 K. The resistivity of the Curie-Weiss metallic Na0.71CoO2 composition has a very low residual resistivity, which attests to the high homogeneity of the crystals prepared by this improved electrochemical method. Our results on the various stable crystal compositions point to the importance of Na ion ordering across the phase diagram.Comment: 9 pages, 9 figure

Size-dependent spinodal and miscibility gaps for intercalation in nano-particles

Using a recently-proposed mathematical model for intercalation dynamics in phase-separating materials [Singh, Ceder, Bazant, Electrochimica Acta 53, 7599 (2008)], we show that the spinodal and miscibility gaps generally shrink as the host particle size decreases to the nano-scale. Our work is motivated by recent experiments on the high-rate Li-ion battery material LiFePO4; this serves as the basis for our examples, but our analysis and conclusions apply to any intercalation material. We describe two general mechanisms for the suppression of phase separation in nano-particles: (i) a classical bulk effect, predicted by the Cahn-Hilliard equation, in which the diffuse phase boundary becomes confined by the particle geometry; and (ii) a novel surface effect, predicted by chemical-potential-dependent reaction kinetics, in which insertion/extraction reactions stabilize composition gradients near surfaces in equilibrium with the local environment. Composition-dependent surface energy and (especially) elastic strain can contribute to these effects but are not required to predict decreased spinodal and miscibility gaps at the nano-scale

Unconventional Charge Ordering in Na0.70CoO2 below 300 K

We present the results of measurements of the dc-magnetic susceptibility chi(T) and the 23Na-NMR response of Na_{0.70}CoO_{2} at temperatures between 50 and 340 K. The chi(T) data suggest that for T > 75 K, the Co ions adopt an effective configuration of Co^{3.4+}. The 23Na-NMR response reveals pronounced anomalies near 250 and 295 K, but no evidence for magnetic phase transitions is found in chi(T). Our data suggest the onset of a dramatic change in the Co 3d-electron spin dynamics at 295 K. This process is completed at 230 K. Our results maybe interpreted as evidence for either a tendency to electron localization or an unconventional charge-density wave phenomenon within the cobalt oxide layer, CoO_2, 3d electron system near room temperature.Comment: 4 pages, 4 figures, re-submitted to Physical Review Letters. The manuscript has been revised following the recommendations of the referees. The discussion section contains substantial change