Softening of the optical phonon by reduced interatomic bonding strength without depolarization.
basic_science · Level V
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- Record sourced from PubMed, PMID 39478211.
- Also identified by DOI 10.1038/s41586-024-08099-0.
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Abstract
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 force<sup>1</sup>, for example, by increasing the Born effective charges<sup>2</sup>. However, it suffers from depolarization effects<sup>3,4</sup> 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. <sup>5</sup>) 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 BaZrO<sub>3</sub> and ultrathin HfO<sub>2</sub> and ZrO<sub>2</sub> films<sup>6,7</sup> grown epitaxially on lattice-mismatched SiO<sub>2</sub>/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.