Elasticity of disordered binary crystals

TL;DR

This paper develops a generalized density-functional approach to calculate elastic constants and phonon dispersion relations in disordered multi-component crystals, including solid solutions with point defects, up to melting.

Contribution

It extends existing methods to handle multi-component disordered crystals, enabling analysis of their elastic and vibrational properties.

Findings

Dispersion relations computed for binary crystals show characteristic signatures.

Acoustic branches become flat in parts of the Brillouin zone.

Crossover between acoustic and optic branches occurs at long wavelengths.

Abstract

The properties of crystals consisting of several components can be widely tuned. Often solid solutions are produced, where substitutional or interstitional disorder determines the crystal thermodynamic and mechanical properties. The chemical and structural disorder impedes the study of the elasticity of such solid solutions, since standard procedures like potential expansions cannot be applied. We present a generalization of a density-functional based approach recently developed for one-component crystals to multi-component crystals. It yields expressions for the elastic constants valid in solid solutions with arbitrary amounts of point defects and up to the melting temperature. Further, both acoustic and optical phonon eigenfrequencies can be computed in linear response from the equilibrium particle densities and established classical density functionals. As a proof of principle, dispersion relations are computed for two different binary crystals: A random fcc crystal as an example for a substitutional, and a disordered sodium chloride structure as an example of an interstitial solid solution. In cases where one of the components couples only weakly to the others, the dispersion relations develop characteristic signatures. The acoustic branches become flat in much of the first Brillouin zone, and a crossover between acoustic and optic branches takes place at a wavelength which can far exceed the lattice spacing.*