The Interplay of Electronic Decoherence, Nonadiabatic Coupling, and the Quasiparticle Band Gap Governs Carrier Dynamics in Lead-Free Carbide Antiperovskites

Photoexcited carrier dynamics are governed by the interplay between electronic structure and thermally driven lattice motion, yet how this interplay shapes hot-carrier relaxation and nonradiative recombination remains unexplored in the emerging class of lead-free carbide antiperovskites. Using ab initio nonadiabatic molecular dynamics, we investigate finite-temperature carrier dynamics in Ca6CSe4 and Sr6CSe4 and reveal a pronounced pathway-selective response to cation substitution. Replacing Ca2+ with the larger Sr2+ cation narrows the direct quasiparticle band gap from 1.66 to 1.22 eV and modifies the magnitude of thermal structural and electronic fluctuations. In the conduction-band manifold, smaller energy separations and stronger energy fluctuations in Ca6CSe4 enhance intraband nonadiabatic couplings (NACs), leading to faster hot-electron cooling. Hot-hole relaxation exhibits a more complex dependence on energy-level separation, NAC strength, and electronic decoherence, with slower near-edge relaxation in Sr6CSe4. Across the fundamental gap, Ca6CSe4 exhibits a wider band gap, ∼53% weaker VBM–CBM NACs, and ∼28% faster electronic decoherence, resulting in a substantially longer nonradiative recombination lifetime of 40.3 ns, compared with 2.2 ns for Sr6CSe4. These results establish a microscopic picture of pathway-selective carrier dynamics in carbide antiperovskites and demonstrate how cation substitution can distinctly reshape intraband hot-carrier cooling and interband nonradiative recombination.