Softening of the optical phonon by reduced interatomic bonding strength without depolarization

Softening of the transverse optical (TO) phonon, which could trigger ferroelectric phase transition, can usually be achieved by enhancing the long-range Coulomb interaction over the short-range bonding force¹, for example, by increasing the Born effective charges². However, it suffers from depolarization effects^3,4 as the induced ferroelectricity is suppressed on size reduction of the host materials towards high-density nanoscale electronics. Here, we present an alternative route to drive the TO phonon softening by showing that the abnormal soft TO phonon in rocksalt-structured ultrawide-bandgap BeO (ref. ⁵) is mainly induced by a substantial reduction in the short-range bonding interaction due to the Be-O bond stretching caused by an electron cloud-overlap-induced Coulomb repulsion between two adjacent oxygen ions that are arranged octahedrally around an extremely small Be ion. We further demonstrate the emergence of robust ferroelectricity in strain-induced perovskite BaZrO₃ and ultrathin HfO₂ and ZrO₂ films^6,7 grown epitaxially on lattice-mismatched SiO₂/Si substrate arising from the softening of the TO phonon driven by a reduction in the short-range bonding strength of biaxial strain-induced stretching bonds. These findings shed light on developing a unified theory for ferroelectricity enhancement in ultrathin films free from depolarization fields by tailoring chemical bonds using ionic radius differences, strains, doping and lattice distortions.