Understanding mechanisms of thermal expansion in complex oxide thin-films from first principles: Role of high-order phonon-strain anharmonicity

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 Grfineisen theory of thermal expansion in combination with density functional calculations and the quasiharmonic approximation to uncover mechanisms of thermal expansion in ferroelectric PbTiO3 thin-films. We show that although there are large changes in the elastic constants and Grfineisen parameters with both misfit strain and temperature, surprisingly the temperature evolution of the structural parameters can be modeled using a simple model of linear elasticity; the only inputs required are the misfit strain and bulk PbTiO3 elastic constants. We show that a near-cancellation between different types of anharmonicity gives rise to this behavior. Our results illustrate how different sources of anharmonicity can affect materials' properties in different ways, even for the same material. The framework used in our study is general and illustrates how different types of anharmonicity can be systematically identified and explored. Published under an exclusive license by AIP Publishing.