Pressure-induced Phase Changes in Cesium Lead Bromide Perovskite Nanocrystals with Planar Defects*

Lead halide perovskites have a rich landscape of structural and optical properties, which can be explored and possibly controlled by applying high pressure. CsPbBr₃ NCs with Ruddlesden-Popper (RP) faults, formed via post-synthetic fusion growth, are significantly larger in size than as-synthesized NCs and display exceptional emission stability. We compare synchrotron-based high pressure X-ray diffraction and photoluminescence (PL) properties of CsPbBr₃ (without RP) and RP-CsPbBr₃ (with RP), and resolve their crystal structure under pressure for the first time. CsPbBr₃ undergoes a phase transition from ambient orthorhombic (S.G: Pnma) to cubic (S.G: Pm-3m) phase at 1.7 GPa and RP-CsPbBr₃ transforms from Pnma to monoclinic (S.G: P2₁/m) phase at 0.74 GPa in addition to several isostructural transitions. Density-functional calculations predict a narrowing of the band gap with pressure, concomitant with the PL energies. The RP-CsPbBr₃ NCs exhibit enhanced PL intensity at 1 GPa and show band gap opening at high pressures. This study opens new strategies for tuning not just the structural properties but also tuning planar defects in alkali lead halide crystals for improved optical properties.