First principles study of the influence of (110)-oriented strain on the ferroelectric properties of rutile TiO$_2$

TL;DR

This study uses first principles calculations to show how (110)-oriented tensile strain can induce ferroelectricity in rutile TiO$_2$, with potential for strain engineering of its dielectric and piezoelectric properties.

Contribution

It reveals the strain-induced ferroelectric instability in rutile TiO$_2$ and discusses how strain can control polarization direction and dielectric response.

Findings

Polar phonon modes soften under tensile (110) strain

Ferroelectric distortion becomes unstable at accessible strain levels

Polarization can exceed that of BaTiO$_3$

Abstract

We use first principles density functional theory to investigate the softening of polar phonon modes in rutile TiO$_2$ under tensile (110)-oriented strain. We show that the system becomes unstable against a ferroelectric distortion with polarization along (110) for experimentally accessible strain values. The resulting polarization, estimated from the Born effective charges, even exceeds the bulk polarization of BaTiO$_3$. Our calculations demonstrate the different strain dependence of polar modes polarized along (110) and (001) directions, and we discuss the possibility of strain engineering the polarization direction, and the resulting dielectric and piezoelectric response, in thin films of TiO$_2$ grown on suitable substrates.