Possible emergence of modulated strained phases in incipient ferroelectrics through flexoelectric phonon coupling

Using a combination of first-principles calculations and analytical modeling, we explore the possibility that ferroelectric (FE) order in certain incipient ferroelectrics can be preempted by the emergence of a modulated strained phase (MSP), which may persist over a narrow parameter range at zero temperature before the onset of long-range FE order. The hybridization between soft optical and acoustic phonon branches in such systems can be strongly enhanced at small but finite momenta, manifesting, in some cases, as deviations from the linear acoustic phonon dispersion. In centrosymmetric crystals, the leading microscopic mechanism for this hybridization is the flexoelectric effect—the polarization induced by strain gradients associated with acoustic phonon propagation at finite momentum. We perform first-principles density-functional perturbation theory (DFPT) calculations for centrosymmetric BaO, ZnO, and TiO2 under biaxial strain and discuss our results within the framework of the long-wavelength theory of flexoelectricity. We identify the material candidates and conditions under which this effect is most likely to be experimentally observed.