Ab initio quasi-harmonic thermoelasticity, piezoelectricity, and thermoelectricity of polar solids at finite temperature and pressure: Application to wurtzite ZnO

TL;DR

This paper extends an ab initio method to study the thermodynamic, elastic, piezoelectric, and thermoelectric properties of polar solids like ZnO at finite temperature and pressure, using DFT and DFPT within the quasi-harmonic approximation.

Contribution

It introduces a generalized formalism for insulators with internal degrees of freedom, enabling comprehensive property calculations under varying conditions.

Findings

Validated the approach on wurtzite ZnO across wide pressure and temperature ranges.

Compared internal degrees of freedom frameworks: ZSISA and FFEM.

Provided temperature-dependent piezoelectric and pyroelectric tensors.

Abstract

We generalize a previously established ab initio approach-originally developed for hexagonal close-packed (hcp) metals-to accommodate solids with both internal and external degrees of freedom. This extension enables the thermodynamic and thermoelastic characterization of insulators, including those with non-vanishing piezoelectric and pyroelectric tensors. Utilizing Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) within the quasi-harmonic approximation, we derive the pressure and temperature dependence of these properties. Specifically, we investigate internal degrees of freedom using two distinct frameworks: the Zero Static Internal Stress Approximation (ZSISA) and Full Free Energy Minimization (FFEM). We then compare these approximations by computing internal and external thermal expansions, as well as temperature-dependent piezoelectric and pyroelectric tensors. Finally, we demonstrate the generalized formalism by calculating the thermodynamic properties of wurtzite ZnO across a broad range of pressures and temperatures.