Chemical, Structural, and Transport Properties of Na1-xCoO2

TL;DR

This study investigates the structural, thermal, and transport properties of Na1-xCoO2 under various pressures and temperatures, revealing complex anisotropic behaviors and electronic transitions relevant for thermoelectric applications.

Contribution

It provides comprehensive measurements of physical properties of Na0.57CoO2 under pressure and temperature, offering new insights into its anisotropic responses and electronic structure.

Findings

Large thermal expansion along the c-axis between 150-250 K

Resistivity transitions from metallic to non-metallic behavior

Phonon thermal conductivity remains unaffected in cold-pressed samples

Abstract

We report measurement of room-temperature compressibility, thermal expansion, thermoelectric power a(T) at various pressures P < 20 kbar, basal-plane resistivity rab (T), magnetic susceptibility and thermal conductivity k(T) taken on single-crystal or cold-pressed Na0.57CoO2. An enhancement of a large thermopower with a change of slope occurs on heating near 100 K, but this enhancement is progressively suppressed by pressure. The c-axis thermal expansion is large in the interval 150 K <T < 250 K where the c-axis resistivity exhibits a smooth transition from a metallic to a non-metallic temperature dependence; but the basal-plane thermal expansion remains negligible for all temperatures T < 300 K. On the other hand, the basal-plane room-temperature compressibility is large in the interval 0 < P < 22 kbar, becoming negligible in the range 22 < P < 45 kbar, whereas the c-axis room-temperature compressibility is anomalously large in the pressure range 22 < P < 35 kbar. The basal plane resistivity is prop. to T^(3/2) below 175 K where there is 3D metallic conduction; it rises less rapidly with temperature where the metallic conduction is confined to 2D. The phonon contribution to the thermal conductivity of a cold-pressed ceramic sample is not suppressed, as previously reported. These findings are rationalized with the aid of the virial theorem, recognition of a pinning of the nominal Co(IV)/Co(III) redox couple at the top of the O2-:2p6 bands, and a schematic location of the a1T and eT antibonding bands of this couple with respect to the Fermi energy.