White-light emission triggered by pseudo Jahn–Teller distortion at the pressure-induced phase transition in Cs4PbBr6.

The zero-dimensional all-inorganic perovskite Cs₄PbBr₆ has been studied under pressure by single-crystal X-ray diffraction, optical absorption, and photoluminescence. The analysis of the experimental results has been supported by calculations of the electronic band structure. The crystal undergoes a sequence of two first-order pressure-induced phase transitions at 2.6 and 3.2 GPa with the space group changes R3¯c → Cmce → I4/mnc, which contradicts the previously reported transition R3¯c →B2/b. Under compression, the crystal bandgap progressively narrows across the three phases, and in a jumpwise manner at the transition points, leading to an overall decrease of the energy gap by more than 0.5 eV at 4.2 GPa. The pressure evolution of the bandgap perfectly correlates with the electronic band structure changes modelled by first-principles calculations using the pressure-dependent structural data. Our structural study revealed an unprecedented pressure-induced increase in the volume of PbBr₆ octahedra, by more than 5%, upon the transition to the tetragonal phase. This distortion, associated with the shortening of two and the lengthening of four Pb–Br distances, has been attributed to the pseudo Jahn–Teller (PJT) effect. Thus, the pressure-induced white-light emission that occurs upon the transition to the tetragonal phase is evidently induced by the PJT distortion. [ABSTRACT FROM AUTHOR]