Relying on the anharmonic special displacement method, we introduce an ab initio quasistatic polymorphous framework to describe local disorder, anharmonicity, and electron-phonon coupling in superionic conductors. Using cubic ${\mathrm{Cu}}_2\mathrm{Se}$, we show that positional polymorphism yields the breakdown of the phonon quasiparticle picture, leading to extremely overdamped anharmonic vibrations while preserving transverse acoustic phonons, consistent with experiments. We also demonstrate highly broadened electronic spectral functions with band gap openings of 1.0 eV due to polymorphism, and that anharmonic electron-phonon coupling leads to a band gap narrowing with increasing temperature. Our approach, relying on generating a handful of configurations, opens the way for efficient calculations in superionic crystals to elucidate their compelling high figure of merit.

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• Anharmonic lattice dynamics
• Electron-phonon coupling
• Electronic structure
• Thermoelectrics
• Density functional theory
• First-principles calculations