Halogen–Halogen Noncovalent Bond Formation Determines the Pressure-Dependent Unique Emission Behavior in Zero-Dimensional Halide Perovskites

Self-trapped exciton (STEs)-driven emission has made zero-dimensional (0D) halide perovskites unique and promising for light-emitting technologies. Strain engineering modulates STEs and shows a unique sharp modulation of emission properties at a particular range of external pressure in 0D all-inorganic perovskites. Using state-of-the-art first-principle calculations on A4BX6 perovskites, we have unveiled the structural origin of this observation. We find that, at a critical pressure range, STE get a sharp stabilization due to the [BX6]4– octahedral twisting and the concomitant emergence of strong halogen–halogen noncovalent interaction between the neighboring octahedra, resulting the formation of axially elongated and equatorially compressed [BX6]4– octahedra, making ns2 lone-pair stereochemically active and facilitating higher extent of electron–hole overlap, resulting in the formation of highly stable well-defined STE. This study offers a comprehensive understanding of strain-induced optoelectronic modulation in 0D perovskites and unveils the origin of the experimental observations on emission enhancement at a critical range of pressure.