Chemical, structural, and transport properties of Na1-xCoO2 -: art. no. 075108

We report measurements of room-temperature compressibility, thermal expansion, thermoelectric power alpha(T) at various pressures Pless than or equal to20 kbar, basal-plane resistivity rho(ab)(T), magnetic susceptibility, and thermal conductivity kappa(T) taken on single-crystal or cold-pressed Na1-xCoO2, (1-x)approximate to0.57. Whereas the c-axis thermal expansion is large, the basal-plane thermal expansion remains negligible for all temperatures T<300 K. We propose here that pinning of the nominal Co(IV)/Co(III) redox couple at the top of the O2-:2p(6) bands, and a schematic location of the a(1)(T) and e(T) antibonding bands of this couple with respect to the Fermi energy are responsible for the large thermoelectric power found in this compound. On the other hand, the phonon contribution to the thermal conductivity measured on a dense cold-pressed ceramic sample is not as small as previously reported.