First-principles study of the influence of (110) strain on the   ferroelectric trends of TiO$_2$
  We investigate the impact of uniaxial strain on atomic shifts, dipolar interactions, polarization and electric permittivity in TiO$_2$ (rutile) by using two different implementations of density functional theory. It is shown that calculations using the Vienna ab inito simulation package (VASP) and the plane-wave self-consistent field method (PWscf) yield qualitatively the same atomic relaxations and ferroelectric trends under strain. The phonon dispersion curves of unstrained and strained TiO$_2$ (rutile) obtained by employing the linear response method confirm previous calculations of the giant LO-TO splitting and the appearance of soft polar modes. A second order phase transition into a ferroelectric phase with polarization along (110) appears under expansive strain in (110) direction. 
Anna Gr\"unebohm, Mario Siewert, Peter Entel and Claude Ederer
cond-mat.mtrl-sci