Phonon calculations were performed using the phonopy package. For all materials, a 10 × 10 × 10 grid was used to sample the phonon Brillouin zone, as this was found to be sufficiently dense to converge band centers to within 0.1 meV. Phonon frequencies (ω qν ) and associated normalized eigenvectors (W qν ) were calculated from an eigendecomposition of the 3N a × 3N a dynamical matrix at each point q in the Brillouin zone. Here, ν denotes the band index, and N a is the number of atoms in the primitive cell. Band centers were calculated as