Structural properties and cation ordering in layered hexagonal CaxCoO2

TL;DR

This study investigates the structural properties and cation ordering in layered hexagonal CaxCoO2, revealing various ordered states, structural distortions, and their relation to superconductivity, through TEM analysis and systematic structural characterization.

Contribution

It provides a detailed analysis of cation ordering and structural phenomena in CaxCoO2, identifying common ordered states and modulations across different compositions.

Findings

Identification of two well-defined cation ordered states at x ≈ 1/2 and x ≈ 1/3.

Observation of multiple ordered states, phase separation, and incommensurate modulations.

Detection of a structural distortion q2 in compositions with x ≤ 0.35, including in superconducting Na0.33CoO2·1.3H2O.

Abstract

A series of CaxCoO2 (0.15 <= x <= 0.40) materials have been prepared by means of ion exchange reaction from NaxCoO2. Transmission electron microscopy (TEM) measurements revealed a rich variety of structural phenomena resulting from cation ordering, structural distortion and twinning. Systematical structural analysis, in combination with the experimental data of NaxCoO2 (0.15 <= x <= 0.8) and SrxCoO2 (1.5 <= x <= 0.4) systems, suggests that there are two common well-defined cation ordered states corresponding respectively to the orthorhombic superstructure at around x = 1/2 and the 31/2a31/2a superstructure at around x = 1/3 in this kind of system. Multiple ordered states, phase separation, and incommensurate structural modulations commonly appear in the materials with 0.33 < x < 0.5. The TEM observations also reveals an additional periodic structural distortion with q2 = a* / 2 in materials for x <= 0.35. This structural modulation also appears in the remarkable superconducting phase Na0.33CoO2 1.3H2O.