Pressure-Induced Structural Phase Transitions in MBANP: A First-Principles Study of Elastic, Phonon, and Optical Properties

<title>Abstract</title> <p>Context: Organic nonlinear optical (NLO) crystals such as 2-(alpha-methylbenzylamino)-5-nitropyridine (MBANP) are pivotal for photonic and optoelectronic technologies, yet their behavior under hydrostatic pressure remains entirely unexplored. This study presents the first comprehensive investigation of MBANP single crystals under hydrostatic pressure up to 20 GPa using first-principles calculations. Our results reveal significant anisotropic compression and a pressure-induced isosymmetric structural transition near 14 GPa. This transition yields a high-pressure phase with more isotropic mechanical properties. Furthermore, pressure markedly enhances intramolecular charge transfer along the donor-acceptor direction, narrows the bandgap from 2.226 eV to 1.056 eV, and induces strongly anisotropic changes in carrier mobility. Analysis of elastic constants confirms mechanical stability across the entire pressure range, with bulk, shear, and Young's moduli all increasing significantly and an inflection in the B/G ratio near 14 GPa correlating with the structural transition. Hirshfeld surface analysis quantitatively reveals a pressure-driven reorganization of intermolecular interactions, where weak van der Waals contacts (H...H, C...H) diminish in favor of strengthened pi-pi stacking (C...C) and hydrogen bonding (N/C-H...O), with certain C-H...O contacts even transforming into genuine hydrogen bonds. Raman spectroscopy reveals uniform blue-shifting of all characteristic vibrational modes, indicating bond compression and enhanced intermolecular constraints. Optical property calculations demonstrate systematic pressure-dependent shifts in the dielectric function, refractive index, and absorption coefficient, with the static refractive index increasing from 1.86 to 2.1. This work elucidates the intricate structure-property relationships of MBANP under extreme conditions, demonstrating pressure as a potent tool for tuning its functional properties and affirming its stability for applications in miniaturized optoelectronic devices. Methods All calculations were performed using the CASTEP code based on density functional theory (DFT). The exchange-correlation energy was described within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) functional, with the TS van der Waals correction applied to account for weak intermolecular interactions. Electron-ion interactions were represented by norm-conserving pseudopotentials, and the plane-wave basis set was expanded to a cutoff energy of 830 eV. Geometry optimizations employed convergence criteria of 5.0E-6 eV/atom for energy, 0.01 eV/Angstrom for forces, 0.02 GPa for stress, and 5.0E-4 Angstrom for displacement. The Brillouin zone was sampled using Monkhorst-Pack grids with a spacing of 0.05 Angstrom^-1 for geometry optimizations and a 38x31x11 k-point mesh for band structure calculations. Phonon dispersion spectra were computed to verify dynamical stability, and Hirshfeld surface analysis was performed using CrystalExplorer.</p>