Unified picture for the pressure-controlled band gap in inorganic halide perovskites: Role of strain-phonon and phonon-phonon couplings

Pressure in halide perovskites attracts extensive attention recently as an effective tool for band-gap engineering. Here, combining first-principles calculations and symmetry-mode analyses, we give a general insight into the role of pressure in inorganic halide perovskites and provide a complete and consistent description of the evolution of band gap that observed in high-pressure experiments. We reveal that strain-phonon and phonon-phonon couplings are the essential factors determining the band-gap evolution. The subtle interplay between strain-phonon and phonon-phonon couplings triggers the increase of out-of-phase tilt at a larger pressure, which results in the simultaneous increase of the band gap. Additionally, we point out that the bond lengths vary continuously, and their nonlinear behaviors originate from strain-phonon coupling instead of the stiffening of the volume. With this knowledge, we propose that epitaxial compressive strain continuously decreases the tilt distortion, and reduction of band gap of 0.5 eV is achieved in CsPbBr3 by 5% compressive strain, which may dramatically enhance the energy conversion efficiency.