Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbOX₂ (X = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl₂ can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl₂ and a reversible ferroelectric-to-paraelectric phase transition in NbOI₂ under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbOX₂ for nonlinear optical device applications.

Structural studies of pyrene have been limited to below 2 GPa. Here, we report on investigations of pyrene up to ~35 GPa using in situ single-crystal synchrotron X-ray diffraction in diamond anvil cells and ab initio calculations. They reveal the phase transitions from pyrene-I to pyrene-II (0.7 GPa), and to the previously unreported pyrene-IV (2.7 GPa), and pyrene-V (7.3 GPa). The structure and bonding analysis shows that gradual compression results in continuous compaction of molecular packing, eventually leading to curvature of molecules, which has never been observed before. Large organic molecules exhibit unexpectedly high conformational flexibility preserving pyrene-V up to 35 GPa. Ab initio calculations suggest that the phases we found are thermodynamically metastable compared to pyrene-III previously reported at 0.3 and 0.5 GPa. Our study contributes to the fundamental understanding of the polymorphism of polycyclic aromatic hydrocarbons and calls for further theoretical exploration of their structure–property relationships.

Benzo[a]pyrene (BaP), C₂₀H₁₂, is a representative of polycyclic aromatic hydrocarbons (PAHs), which are ubiquitous in nature and the universe, where they are subjected to extreme conditions. This paper reports the results of investigations of the high-pressure behavior of BaP up to 28 GPa using in situ synchrotron single-crystal X-ray diffraction. We identified two previously unknown polymorphs, BaP-II (P2₁/c) at 4.8 GPa and BaP-III (P 1) at 7.1 GPa. The structural transformation from BaP-I (P2₁/c) to BaP-II (P2₁/c) manifests as an abrupt change in the intermolecular angle and the unit-cell parameters a and b, whereas the transformation from BaP-II (P2₁/c) to BaP-III (P 1) is characterized by a decrease in symmetry. According to density functional theory calculations, BaP-III is the most stable phase above 3.5 GPa. These studies advance our understanding of the structural dynamics and stability of PAHs under high pressure.

In this study, we explored the behavior of naphthalene and anthracene under compression to ∼50 and ∼42 GPa, respectively, using synchrotron single-crystal X-ray diffraction (SCXRD) in diamond anvil cells. Both compounds demonstrate remarkable structural stability, with no phase transitions being observed. The equations of states were obtained with the following parameters: V ₀ = 182.2(2) ų, K ₀ = 8.4(10) GPa, and K' = 5.5(6) for naphthalene, and V ₀ = 238.0(2) ų, K ₀ = 8.4(5) GPa, and K' = 8.0(4) for anthracene. Theoretical calculations correctly reproduce experimental data and enable accurate localization of hydrogen atoms. The analysis of Hirshfeld surfaces for both naphthalene and anthracene suggests that the herringbone packing motif of the molecules, the limited contribution of stronger C···C interactions to intermolecular bonding, and the high flexibility of relatively weak H···H interactions allow a gradual compaction of their structures without phase transitions. Our research contributes to the understanding of the compressional mechanisms, bonding evolution, and structural stability of polyaromatic hydrocarbons under compression.