Pressure-induced band gap closing of lead-free halide double perovskite (CH₃NH₃)₂PtI₆

Lead-free halide double perovskites have recently attracted significant attention due to their exceptional stability and favorable band gaps, making them promising candidates for solar cell applications. However, the relationship between their structural characteristics and intrinsic band gap remains under-explored. This study presents a method to investigate the structure-band gap correlation in a typical halide double perovskite, MA₂PtI₆ (${\mathrm{MA}}^{+}={\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$), using high pressure techniques. The band gap of MA₂PtI₆ is effectively reduced at two different rates of 0.063 eV/GPa and 0.079 eV/GPa before and after 1.2 GPa, and progressively closes as pressure further increases. These optical changes are closely related to the pressure induced structural evolution of MA₂PtI₆. Moreover, a phase transition from trigonal (R-3m) to monoclinic (P2/m) occurs at 1.2 GPa and completes by 2.0 GPa, driven by pressure-induced distortion of the PtI₆²⁻ octahedra, which is responsible for the variation of the band gap. These promising findings pave the way for potential applications in the structural and band gap tuning of halide double perovskites.