Strain game revisited for complex oxide thin-films: Substrate-film thermal expansion mismatch in PbTiO$_3$

TL;DR

This study demonstrates that thermal expansion mismatch significantly influences the properties of ferroelectric PbTiO$_3$ thin-films, and ignoring it leads to qualitatively incorrect predictions of transition temperatures and structural behavior.

Contribution

It systematically investigates the role of substrate thermal expansion mismatch in ferroelectric thin-films using first-principles calculations, highlighting its importance for accurate modeling.

Findings

Thermal expansion mismatch alters ferroelectric transition temperatures.

Ignoring thermal effects leads to qualitatively different structural predictions.

Misfit strain as a single number is ill-defined for PbTiO$_3$.

Abstract

The sensitivity of materials properties, particularly those of perovskite oxides, to epitaxial strain has been exploited to great advantage to create materials with new or enhanced properties. Although it has certainly been recognized that mismatch in the thermal expansion coefficients of the bulk and substrate material will contribute to the misfit strain, the significance of this contribution for ferroelectric perovskite thin-films has not been systematically explored. We use first-principles density functional theory and the example of ferroelectric PbTiO$_3$ thin-films on various substrates to show that ignoring the thermal expansion of the substrate (that is, assuming that the in-plane lattice parameter of the film remains roughly constant as a function of temperature) results in ferroelectric transition temperatures and structural trends that are completely qualitatively different from calculations in which thermal expansion mismatch is properly taken into account. Our work suggests that the concept of a misfit strain defined as a single number is particularly ill-defined for PbTiO$_3$ and invites further study of the interplay between thermal expansion mismatch and structural and functional properties in other thin-film materials.