Understanding mechanisms of thermal expansion in PbTiO$_3$ thin-films from first principles: role of high-order phonon-strain anharmonicity

TL;DR

This study uses first-principles calculations to reveal that high-order phonon-strain anharmonicity significantly influences the thermal expansion of PbTiO$_3$ thin-films, despite their apparent elastic behavior.

Contribution

It uncovers the critical role of high-order anharmonic effects in thermal expansion mechanisms of PbTiO$_3$ thin-films, a novel insight in first-principles materials modeling.

Findings

PbTiO$_3$ thin-films exhibit strong anharmonicity with temperature.

Elastic constants and Gr"{u}neisen parameters vary significantly with strain and temperature.

A near-cancellation of anharmonic effects explains the apparent elastic behavior.

Abstract

The thermal properties of materials are critically important to various technologies and are increasingly the target of materials design efforts. However, it is only relatively recent advances in first-principles computational techniques that have enabled researchers to explore the microscopic mechanisms of thermal properties, such as thermal expansion. We use the Gr\"{u}neisen theory of thermal expansion in combination with density functional calculations and the quasiharmonic approximation to uncover mechanisms of thermal expansion in PbTiO$_3$ thin-films in terms of elastic and vibrational contributions to the free energy. Surprisingly, we find that although the structural parameters of PbTiO$_3$ thin-films evolve with temperature as if they are dominated by linear elasticity, PbTiO$_3$ thin-films are strongly anharmonic, with large changes in the elastic constants and Gr\"{u}neisen parameters with both misfit strain and temperature. We show that a fortuitous near-cancellation between different types of anharmonicity gives rise to this behavior. Our results illustrate the importance of high-order phonon-strain anharmonicity in determining the temperature-dependent structural parameters of PbTiO$_3$ thin-films, and highlight the complex manner in which thermal expansion, misfit strain and elastic and vibrational properties are intertwined.