Phonon Decoupling in Double Perovskite Oxides

In general, a slight displacement of an atom from its equilibrium position affects the forces on the rest of the atoms simultaneously. That's why the phonon has been considered a collective excitation in the periodic arrangement of atoms in solids. Here, using first-principles density functional theory calculations, we first demonstrate that, contrary to typical collective phonons, oxygen-octahedral and oxygen-tetrahedral phonons can be fully decoupled across the entire phonon spectrum, especially in double perovskite oxides. This decoupled oxygen tetrahedral phonon would be strongly localized and can greatly induce band flattening along a specific crystal orientation, enabling unusual site-selective control of the unit cell width domain wall.