Pressure-Driven Phase Transition in Two-Dimensional Perovskite MHy₂PbBr₄

The application of high pressure allows tuning physicochemical properties materials by changing interatomic distances. Pressure may also induce structural phase transitions into new phases with enhanced or novel functional properties. Here, we report complementary high-pressure single-crystal X-ray diffraction, Raman spectroscopy, and optical studies a two-dimensional (2D) perovskite, MHy2PbBr4, comprising very small spacer cation (methylhydrazinium, MHy+). This crystal exhibits highly desired ferroelectric extraordinary multiple linear nonlinear (NLO) Single-crystal diffraction shows that MHy2PbBr4 undergoes an unusual Pmn21 ? P21 transition near 4 GPa, associated the extrusion some MHy+ cations from interlayer space voids located within inorganic sheets, not reported for any 2D hybrid perovskite. transport counter leads to significant increase Pb–NH2 interactions, unprecedented threefold positive compressibility perpendicular polyanionic layers large negative ?22.39 TPa–1 layers. data confirm association strong distortion structure reorganization hydrogen bond network, while absorption spectra compressed ambient-pressure show band gap narrowing, followed its widening in phase. A similar change dependence red shift blue is observed free-exciton (FE) photoluminescence (PL). Furthermore, pressure-induced giant enhancement PL intensity, especially pronounced broad-band emission attributed self-trapped excitons (STEx). We attribute effects, spectra, shortening Pb–Br bonds ambient increased tilts PbBr6 octahedra Overall, our results compound extend understanding effect on perovskites general demonstrate different behavior under compression compared analogues organic cations. They revealed structure–strain mechanism can be used engineering structural, mechanical, optoelectronic