Local Structure of Thermoelectric Ca3Co4O9

TL;DR

This study combines local structural measurements and first principles calculations to model the complex layered structure of Ca3Co4O9, revealing disorder in the rock salt layer and magnetic correlations in the CoO2 layers.

Contribution

It provides a detailed three-dimensional local structure model of Ca3Co4O9, integrating experimental and computational approaches to understand its atomic and magnetic properties.

Findings

Low Co(2)-O coordination with static disorder in rock salt layer

Normal Co(1)-Co(1) correlations above 75K with small disorder

Enhanced magnetic correlations at the onset of magnetic order

Abstract

We have combined temperature dependent local structural measurements with first principles density functional calculations to develop a three dimensional local structure model of the misfit system [Ca2CoO3][CoO2]1.61 (referred to as Ca3Co4O9) which has a rock salt structure stacked incommensurately on a hexagonal CoO2 lattice. The local structural measurements reveal a low coordination of Co(2)-O bonds in the rock salt layer with large static structural disorder. The temperature dependence of the Co(1)-Co(1) bond correlations in the CoO2 layer are found to be normal above ~75K and with a very small static disorder component. An anomalous enhancement in the Co(1)-Co(1) correlations occurs at the onset of long-range magnetic order. Density functional computations suggest that the reduction of the coordination of Co(2) is due to the formation of chains of Co(2)Ox in the a-b plane linked to the Ca-O layers by c-axis Co(2)-O bonds. The reduced dimensionality introduced by the chain-like structure in the rock salt layer and high atomic order in the C