Relating chemical bonding to physical properties: The origin of unexpected isotropic properties in layered materials

TL;DR

This paper reveals that certain layered materials, like Mg3Sb2, have nearly isotropic properties due to a 3D bonding network, challenging the common belief of their anisotropy caused by weak interlayer interactions.

Contribution

It uncovers the nearly isotropic 3D bonding network in Mg3Sb2 and related compounds, explaining their isotropic properties through chemical bonding analysis.

Findings

Mg3Sb2 has a nearly isotropic 3D bonding network.

A parameter based on electron density indicates thermal conductivity anisotropy.

The bonding features are applicable to many Mg-containing layered compounds.

Abstract

Layered materials span a very broad range of solids ranging from van der Waals materials to highly complex crystal structures such as clays. They are commonly believed to have highly anisotropic properties, which is essentially attributed to weak interlayer interactions. The layered Mg3Sb2 structure is currently being intensely scrutinized due to its outstanding thermoelectric properties. Based on quantitative chemical bonding analysis we unravel that Mg3Sb2 exhibits a nearly isotropic three-dimensional (3D) bonding network with the interlayer and intralayer bonds being surprisingly similar, and these unique chemical bonding features are the origin of the nearly isotropic structural and thermal properties. The isotropic 3D bonding network is found to be broadly applicable to many Mg-containing compounds with the layered CaAl2Si2-type structure. Intriguingly, a parameter based on the electron density can be used as an indicator measuring the anisotropy of lattice thermal conductivity in layered structures. This work extends our understanding of structure and properties based on chemical bonding analysis, and it will guide the search for, and design of, layered materials with tailored anisotropic properties.