First-principles study of the influence of (110)-oriented strain on the ferroelectric properties of rutile TiO2

We use first-principles density-functional theory to investigate the softening of polar phonon modes in rutile TiO2 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 BaTiO3. 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 TiO2 grown on suitable substrates.