New Misfit-Layered Cobalt Oxide (CaOH)1.14CoO2

TL;DR

This study reports the discovery and detailed characterization of a new layered cobalt oxide, CaOH)1.14CoO2, revealing its unique crystal structure, conduction mechanisms, and electronic properties influenced by structural modulation and electron correlation.

Contribution

The paper introduces a novel cobalt oxide compound synthesized via high-pressure techniques, with comprehensive structural and electronic analysis highlighting its unique incommensurate layered structure and conduction behavior.

Findings

Layered structure with incommensurate modulation identified.

2D variable-range hopping conduction observed at low temperatures.

Strong electron correlation indicated by Wilson ratio ~2.8.

Abstract

We found a new cobalt oxide (CaOH)1.14CoO2 by utilizing the high-pressure technique. X-ray and electron diffraction studies revealed that the compound has layer structure which consists of CdI2-type CoO2 layers and rock-salt-type double CaOH atomic layers. The two subcells have incommensurate periodicity along the a-axis, resulting in modulated crystal structure due to the inter-subcell interaction. The structural modulation affects carrier conduction through the potential randomness. We found that the two-dimensional (2-D) variable-range hopping (VRH) regime with hole conduction is dominant at low temperature for this compound, and that the conduction mechanism undergoes crossover from the 2-D VRH regime to thermal activation-energy type one with increasing temperature. Based on the experimental results of resistivity, thermoelectric power, magnetic susceptibility and specific heat measurements, we suggested a possible electronic-band structure model to explain these results. The cobalt t2g-derivative band crosses Fermi energy level near the band edge, yielding small finite density of localized states at the Fermi level in the band. The observed resistivity, Seebeck coefficient, large Pauli paramagnetic component in the magnetic susceptibility and comparatively small Sommerfeld constant in the specific heat are principally attributed to the holes in the t2g-derivative band. We estimated the Wilson ratio to be about 2.8, suggesting the strong electron correlation realized in this compound.