Tuning nonradiative recombination via cation substitution in inorganic antiperovskite nitrides

Inorganic antiperovskite nitrides have recently emerged as promising materials for photovoltaic applications, yet their nonradiative recombination dynamics remain largely unexplored. Here, we examine the influence of X-site cation substitution on the nonradiative electron-hole recombination in ${\mathrm{X}}_{3}\mathrm{NSb}$ $(\mathrm{X}=\mathrm{Ca}, \mathrm{Sr}, \text{and} \mathrm{Ba})$. Ca- and Sr-based compounds adopt a cubic phase, whereas Ba stabilizes in a hexagonal structure, introducing pronounced symmetry-driven effects. To separate symmetry effects from cation chemistry, we also examine the hexagonal polymorph of ${\mathrm{Sr}}_{3}\mathrm{NSb}$ $({\mathrm{Sr}}_{3}{\mathrm{NSb}}_{\mathrm{hexa}})$. Substituting Ca with Sr narrows the band gap, suppresses octahedral and band-edge fluctuations, reduces nonadiabatic (NA) coupling by $\ensuremath{\sim}54%$, and extends carrier lifetimes by a factor of 2.5. In ${\mathrm{Sr}}_{3}{\mathrm{NSb}}_{\mathrm{hexa}}$, the combination of larger band gap and enhanced band gap fluctuations---leading to faster dephasing---further slows down recombination by $41%$. In contrast, in ${\mathrm{Ba}}_{3}{\mathrm{NSb}}_{\mathrm{hexa}}$, enhanced NA coupling accelerates recombination relative to ${\mathrm{Sr}}_{3}{\mathrm{NSb}}_{\mathrm{hexa}}$. Overall, recombination lifetimes are dictated by the interplay between band gap, NA coupling strength, and decoherence time, with ${\mathrm{Sr}}_{3}{\mathrm{NSb}}_{\mathrm{hexa}}$ exhibiting the longest lifetime. These findings highlight the coupled influence of cation chemistry and crystal symmetry in tailoring carrier dynamics for high-performance antiperovskite-based optoelectronics materials.