Pressure-Induced Structural, Mechanical, and Optoelectronic Tunability in Fluorinated Two-Dimensional Hybrid Perovskite (FPMA) ₂ PbI ₄

We report a systematic high-pressure investigation of fluorinated two-dimensional hybrid perovskite (FPMA)₂PbI₄, combining <i>in situ</i> synchrotron powder X-ray diffraction (HP-PXRD), photoluminescence spectroscopy (HP-PL), and first-principles density functional theory (DFT) simulations. Under hydrostatic compression to 5.54 (±0.4) GPa, HP-PXRD exhibits anisotropic lattice contraction (<i>a/a</i>_o = 4.54%, <i>b/b</i>_o = 3.35%, <i>c/c</i>_o = 9.79%), leading to an isothermal bulk modulus of 15.6 (±1.3) GPa (<i>K</i>₀ = 29.9 TPa^-1) and incipient amorphization. DFT-optimized geometries confirm progressive PbI₆ octahedral flattening, concomitant to interlayer contraction, and intensification of F···I, F···H, and I···H interactions. Full elastic tensor analysis reveals moderate anisotropy (<i>A</i>_E = 2.09, <i>A</i>_G = 2.40), along with a broad Poisson's ratio of (-0.28 to 0.63), resulting from cooperating auxetic and conventional elastic deformations of the channels. HP-PL measurements show a continuous red shift to ∼616.9 from 535.4 nm, accompanied by progressive intensity quenching, which is ascribable to bond-contraction-driven bandgap reduction and pressure-amplified nonradiative recombination within a partially amorphized matrix. DFT band-structure computations reproduce pressure-dependent direct-gap narrowing, retaining the collinearity of the <b><i>k</i></b>-space valence-band maximum (VBM) and conduction-band minimum (CBM). Overall, this study demonstrates coupled structural, mechanical, and optoelectronic tunability in (FPMA)₂PbI₄, which underscores its potential for strain-engineered optoelectronic applications and flexible photonic systems with pressure-tunable emission across the visible range.