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

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

Heavy metals in South America aerosol during 20th\mathsf{^{th}} century from Illimani ice-core, Eastern Bolivian Andes


A 137 m ice-core drilled from Eastern Bolivian Andes at Nevado Illimani ($16^\circ37'$ S, $67^\circ46'$ W, 6350 m asl) allows studying historical concentrations of heavy metals in South America aerosol along the 20$^{\rm th}$ century. About 50 m of this ice-core were dated by multi-proxy analysis, providing a record of environmental variations of about 80 years from 1919 to 1999. Elemental concentrations for 45 chemical species (Li to U) in 744 samples along the upper 50 m ice-core section were determined by Inductively Coupled Plasma-Mass Spectrometry. This work focuses on heavy metal concentrations along the ice-core profile. Further discussion on other trace elements is given elsewhere [1]. Most elements like Al, Fe, La, Th and U show a pattern for concentration averages and standard deviations with little variation along 20$^{\rm th}$ century, evidencing their natural origin. Other heavy metals like: Pb, Mo, Zn, Cd, Ni, Co and Cu show increases in average concentrations and standard deviations from the beginning of 20$^{\rm th}$ century to recent years. For example, Cu average concentration values change from $0.604\pm0.440$ ng g$^{-1}$ to $3.46\pm3.02$ ng g$^{-1}$ along 20$^{\rm th}$ century. Time evolution of these elements concentrations' may be related to local economic growth and human activities in South America.