Microscopic origin of optical-phonon-dominated lattice thermal transport in A-site ordered X₆N₂SbBi (X = Mg, Ca, Sr) double antiperovskites

Quaternary A-site anion-ordered double antiperovskites offer a structurally rich platform for tuning optoelectronic properties, yet their phonon thermal-transport mechanisms remain poorly understood. Here, we systematically investigate the lattice dynamics and phonon thermal transport of X₆N₂SbBi (X = Mg, Ca, Sr) using first-principles calculations, self-consistent phonon theory, and the phonon Boltzmann transport equation including both three- and four-phonon scattering. We find that optical phonons account for 50%–53%, 63%–68%, and 65%–72% of the lattice thermal conductivity (κ_L) in Mg₆N₂SbBi, Ca₆N₂SbBi, and Sr₆N₂SbBi, respectively. Cumulative κ_L and atom-projected phonon spectra identify mid-frequency X-dominated optical modes as the principal heat-carrying channels. The X atoms occupy face-centered sites and serve as shared vertices of neighboring N-centered X₆ octahedra, forming a connected framework. Shallow X-site potential wells enable large-amplitude vibrations, while the interconnected framework couples them across neighboring octahedra, producing numerous dispersive, spatially extended mid-frequency optical modes whose combined contribution exceeds that of the acoustic branches. By contrast, high-frequency N-dominated optical modes contribute less to κ_L because their low participation ratios indicate strong localization, despite their relatively high group velocities. Finite-temperature phonon renormalization and anharmonic-force analyses further reveal pronounced lattice anharmonicity, and four-phonon scattering substantially suppresses κ_L. Besides, the κ_L follows the order Mg₆N₂SbBi > Ca₆N₂SbBi > Sr₆N₂SbBi and are consistently larger values along the a axis than along the c axis. These results elucidate the origin of optical-phonon-dominated thermal transport in X₆N₂SbBi and highlight the importance of optical modes in governing heat transport in strongly anharmonic quaternary double antiperovskites.