Fully analytic valence force field model for the elastic and inner elastic properties of diamond and zincblende crystals

TL;DR

This paper develops three analytical methods to determine valence force field parameters for diamond and zincblende crystals, incorporating electrostatic effects to accurately model elastic and inner elastic properties across different material types.

Contribution

The paper introduces new analytical approaches for deriving valence force field parameters that account for ionic character and electrostatic interactions, improving modeling accuracy for various crystal materials.

Findings

Models accurately describe structural relaxation in superlattices and alloys.

Inclusion of electrostatic terms stabilizes the model for ionic materials.

The models reproduce key phonon band structure features.

Abstract

Using a valence force field model based on that introduced by Martin, we present three related methods through which we analytically determine valence force field parameters. The methods introduced allow easy derivation of valence force field parameters in terms of the Kleinman parameter $\zeta$ and bulk properties of zincblende and diamond crystals. We start with a model suited for covalent and weakly ionic materials, where the valence force field parameters are derived in terms of $\zeta$ and the bulk elastic constants $C_{11}$, $C_{12}$, and $C_{44}$. We show that this model breaks down as the material becomes more ionic and specifically when the elastic anisotropy factor $A = 2C_{44}/(C_{11}-C_{12}) > 2$. The analytic model can be stabilised for ionic materials by including Martin's electrostatic terms with effective cation and anion charges in the valence force field model. Inclusion of effective charges determined via the optical phonon mode splitting provides a stable model for all but two of the materials considered (zincblende GaN and AlN). A stable model is obtained for all materials considered by also utilising the inner elastic constant $E_{11}$ to determine the magnitude of the effective charges used in the Coulomb interaction. Test calculations show that the models describe well structural relaxation in superlattices and alloys, and reproduce key phonon band structure features.