Resistivity and Thermoelectric power of NaxCoO2 (x =1.0, 0.7 and 0.6) system

TL;DR

This study investigates the electrical resistivity and thermoelectric power of NaxCoO2 compounds with varying sodium content, revealing metallic and semiconducting behaviors and significant electronic structure changes related to sodium levels.

Contribution

It provides detailed measurements of thermoelectric and resistivity properties across different sodium compositions, highlighting the impact of Na content on electronic behavior and thermoelectric efficiency.

Findings

x=1.0 is metallic down to 5K with high thermoelectric power

x=0.6 is semiconducting with sign-changing thermopower

Significant electronic structure changes occur with decreasing Na content

Abstract

Results of thermo-electric power (S) and electrical resistivity (r) measurements are reported on NaxCoO2 compounds with x = 1.0, 0.7 and 0.6. These are single-phase compounds crystallizing in the hexagonal structure (space group P63/mmc) at room temperature. Thermo-electric power values at 300K (S300K) are, 80mV/K, 39mV/K and 37mV/K for x = 1.0, 0.7 and 0.6 samples, respectively. The samples with x=0.7 and 1.0 are metallic down to 5 K, while the x = 0.6 sample is semiconducting. The value of r300K for x = 1.0 sample is \~0.895 mW-cm and the power factor (S2/r) is = 7.04 x 10-3 W/mK2 which qualifies it as a good thermo-electric material. In x =1.0 sample, S(T) is positive throughout 300-5K temperature range and decreases monotonically to zero as temperature T= 0. In contrast, S(T) of x = 0.7 and 0.6 samples changes sign and shows negative values between 90 K and 16 K before approaching zero as T = 0. Anomalous S(T) behavior of x = 0.6 and 0.7 samples, which are coincidentally the precursor materials to the reported superconductivity in this class of materials, indicates a dramatic change in the electronic structure of these compounds on lowering the Na content.