Pressure-Driven Structural and Optoelectronic Tuning of Cl-Substituted 2D Lead Halide Perovskite (ClPMA)₂PbI₄

We present a systematic high-pressure investigation of the chlorine-functionalized two-dimensional hybrid perovskite (ClPMA)₂PbI₄, integrating <i>in situ</i> high-pressure synchrotron powder X-ray diffraction (HP-PXRD), photoluminescence spectroscopy (HP-PL), and first-principles density functional theory (DFT) calculations. Under hydrostatic compression up to 6.18 (±0.42) GPa, HP-PXRD reveals anisotropic lattice contraction (Δ<i>a/a</i>₀ = 4.06%, Δ<i>b/b</i>₀ = 3.00%, Δ<i>c/c</i>₀ = 8.66%) with a bulk modulus of 16.8 (±1.5) GPa (<i>K</i>₀ = 34.1 TPa^-1) and the onset of amorphization near 6.18 (±0.42) GPa. DFT-optimized structures corroborate progressive PbI₆ octahedral flattening leading to reduced interlayer spacing and enhanced Cl···I, Cl···H, and I···H interactions. The full elastic tensor indicates moderate anisotropy (<i>A</i>_E = 2.1, <i>A</i>_G = 2.63) yet large Poisson's ratios (-0.196 to 0.67), unveiling coexisting auxetic and elastic deformation pathways. HP-PL spectra exhibit a continuous red shift from 525.2 nm to ∼630.5 nm and intensity quenching, attributable to bond-contraction-induced bandgap narrowing and pressure-enhanced nonradiative recombination in the partially amorphous matrix. DFT band-structure calculations confirmed the pressure-dependent direct-gap evolution and maintaining k-space valence-band maximum and conduction-band minimum alignment. These findings elucidate the structural, mechanical, and optoelectronic tunability of (ClPMA)₂PbI₄, underscoring its promise for strain-engineered optoelectronic devices.