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

Technical Summary: Pressure-Induced Structural Phase Transitions in MBANP

Problem Statement
Organic nonlinear optical (NLO) crystals, specifically 2-(α-methylbenzylamino)-5-nitropyridine (MBANP), are critical for next-generation photonic and optoelectronic technologies due to their high second-harmonic generation (SHG) efficiency and structural flexibility. While MBANP has been extensively characterized regarding its temperature-dependent behavior and intermolecular interactions, its response to hydrostatic pressure remains entirely unexplored. Understanding this response is fundamental for elucidating how noncovalent interactions govern crystal stability and NLO activity, and practically essential for assessing the mechanical stability of these materials in miniaturized devices subject to stress.

Methodology
This study employs a first-principles approach based on Density Functional Theory (DFT) using the CASTEP code. The calculations utilize the Generalized Gradient Approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional, augmented by the Tkatchenko–Scheffler (TS) van der Waals correction to accurately model weak intermolecular interactions. Norm-conserving pseudopotentials and a plane-wave basis set with a cutoff energy of 830 eV were used. The study systematically investigates MBANP single crystals under hydrostatic pressure ranging from 0 to 20 GPa. Key analyses include:

• Structural Evolution: Monitoring lattice parameters and molecular conformation.
• Elastic Properties: Calculating elastic constants ($C_{ij}$) and deriving mechanical moduli (Bulk, Shear, Young's, Poisson's ratio) via Voigt-Reuss-Hill averaging, alongside Born stability criteria checks.
• Electronic Structure: Analyzing band structures, density of states (PDOS), charge density, electron localization function (ELF), and charge transfer mechanisms.
• Intermolecular Interactions: Utilizing Hirshfeld surface analysis and 2D fingerprint plots to quantify contact types.
• Vibrational and Optical Properties: Simulating Raman spectra and calculating frequency-dependent optical metrics (dielectric function, refractive index, absorption, reflectivity).

Key Contributions and Results

1. Structural Phase Transition at ~14 GPa:
   The study identifies a pressure-induced isosymmetric structural transition near 14 GPa. Below this threshold, the crystal exhibits pronounced anisotropic compressibility ($a$-axis > $c$-axis > $b$-axis). At 14 GPa, an anomalous contraction of the $a$-axis is accompanied by a discontinuous expansion of the $b$- and $c$-axes and a jump in the $\beta$ angle. This signifies a concerted molecular rearrangement or shear-like distortion, resulting in a high-pressure phase with quasi-isotropic mechanical properties. Despite these changes, the space group symmetry ($P2_1$) is preserved.

2. Mechanical Stability and Stiffening:
   MBANP remains mechanically and dynamically stable throughout the 0–20 GPa range, confirmed by the satisfaction of modified Born stability criteria and the absence of imaginary frequencies in phonon dispersion spectra. The material undergoes progressive stiffening, with the Bulk Modulus increasing from 10.7 GPa to 105.2 GPa and the Shear Modulus from 3.76 GPa to 27.2 GPa. The Poisson's ratio ($\nu$) increases monotonically from 0.343 to 0.4 in the 0–12 GPa range before stabilizing around 0.38, indicating a ductile nature. The $B/G$ ratio exhibits significant variation, increasing from 2.84 to 4.7 between 0 and 12 GPa, then decreasing to 3.86 at 20 GPa, with an inflection point at 14 GPa correlating with the structural transition.

3. Electronic and Charge Transfer Enhancements:
   Pressure significantly enhances intramolecular charge transfer (CT) along the donor-acceptor axis (methylbenzylamino to nitro group). The bandgap narrows from 2.226 eV at 0 GPa to 1.056 eV at 20 GPa, driven by the broadening of the valence band (pyridine $\pi$-orbitals) and conduction band (nitro $\pi^*$ states). Charge density analysis reveals increased electron depletion at the donor nitrogen and accumulation at the acceptor oxygen, alongside a shortened N–H⋯O hydrogen bond length, confirming a strengthened "push-pull" effect.

4. Reorganization of Intermolecular Interactions:
   Hirshfeld surface analysis quantifies a pressure-driven shift in interaction dominance. Weak, isotropic van der Waals contacts (H⋯H and C⋯H) decrease in contribution, while strong, directional interactions increase. Notably, C⋯C contacts (indicative of $\pi$–$\pi$ stacking) surge, and specific C–H⋯O contacts transform into bona fide hydrogen bonds (distances < 2.6 Å) above 14 GPa. This reorganization is identified as a key mechanism for maintaining stability under extreme compression.

5. Vibrational and Optical Responses:
   Raman spectra exhibit a uniform blue-shift across all modes, attributed to bond compression and increased force constants. A distinct discontinuity in peak intensities and broadening occurs at 14 GPa, corroborating the phase transition. Optically, the static dielectric constant and refractive index increase with pressure (rising from ~3.56 to ~4.4 and ~1.86 to ~2.1, respectively). However, the absorption edge and main dielectric function peaks shift to higher energies (blue-shift), consistent with the conclusion that the absorption edge shifts to higher energy. Consequently, the material's absorption in the visible region weakens, leading to a decrease in transmittance (from ~87% to ~73% for a 100 nm sample) and an increase in reflectivity.

Significance
This work provides the first comprehensive theoretical investigation of MBANP under high pressure, filling a critical gap in the understanding of its structure-property relationships. The study demonstrates that hydrostatic pressure acts as a potent tool for tuning the functional properties of organic NLO crystals without altering their symmetry. By elucidating the mechanisms of mechanical stiffening, charge transfer enhancement, and intermolecular reorganization, the findings affirm the stability of MBANP for applications in miniaturized optoelectronic devices and offer guidance for the design of next-generation molecular crystals with tunable electromechanical and optical characteristics.