The application of high pressure allows tuning physicochemical properties of materials by changing interatomic distances. Pressure may also induce structural phase transitions into new phases with enhanced or novel functional properties. Here, we report complementary high-pressure single-crystal X-ray diffraction, Raman spectroscopy, and optical studies of a two-dimensional (2D) perovskite, MHy 2 PbBr 4 , comprising a very small spacer cation (methylhydrazinium, MHy + ). This crystal exhibits highly desired ferroelectric and extraordinary multiple linear and nonlinear optical (NLO) properties. Single-crystal X-ray diffraction shows that MHy 2 PbBr 4 undergoes an unusual Pmn2 1 → P2 1 phase transition near 4 GPa, associated with the extrusion of some MHy + cations from the interlayer space into voids located within the inorganic sheets, not reported for any 2D hybrid perovskite. The transport of counter cations leads to a significant increase of Pb− NH 2 interactions, an unprecedented threefold increase of positive linear compressibility perpendicular to the polyanionic layers and a large negative linear compressibility of −22.39 TPa −1 within the layers. The Raman data confirm the association of the phase transition with strong distortion of the crystal structure and reorganization of the hydrogen bond network, while the absorption spectra of the compressed ambient-pressure Pmn2 1 phase show the band gap narrowing, followed by its widening in the highpressure P2 1 phase. A similar change in the pressure dependence from a red shift to a blue shift is also observed for the free-exciton (FE) photoluminescence (PL). Furthermore, the pressure-induced phase transition leads to a giant enhancement of PL intensity, especially pronounced for the broad-band emission attributed to the self-trapped excitons (STEx). We attribute the effects, observed in absorption and PL spectra, to the shortening of Pb−Br bonds in the ambient pressure phase and increased distortion of the inorganic layers and tilts of PbBr 6 octahedra in the high-pressure phase. Overall, our results for a 2D hybrid compound comprising very small spacer cations extend the understanding of the pressure effect on the properties of 2D hybrid perovskites in general and demonstrate a very different behavior under compression compared to the analogues with large organic cations. They revealed that the structure−strain mechanism can be used for engineering new high-pressure phases with unusual structural, mechanical, and optoelectronic properties.
External stimuli trigger conformational up-conversion in the molecules of di-p-tolyl disulfide (CH3–C6H4–S−)2, compensating the stress and absorbing its energy. These mechanochemical transformations explain at the molecular level the lubricating performance of di-p-tolyl disulfide and suggest new applications for storing energy. High pressure induces the conversion either in the solid state, as a phase transition, or at the nucleation stage on the solution-solid interphase as a detour of crystallization preference. The discontinuous transition at 1.6 GPa reduces the symmetry of ambient-pressure monoclinic phase α (space group P21) to triclinic phase β (space group P1). The recrystallizations above 0.45 GPa yield a new polymorph γ (space group P21 /c), most spectacular in the conformational up-conversion absorbing 6 kJmol–1.
This work describes, for the first time, the application of combined pressure and temperature stimuli in disulfide metathesis reactions. In the system studied, above a pressure of 0.2 GPa, equimolar amounts of symmetric disulfides bis 4-chlorophenyl disulfide [(4-ClPhS) ] and bis 2-nitrophenyl disulfide [(2-NO PhS) ] react to give the heterodimeric product 4-Cl-PhSSPh-2-NO . In contrast to experiments conducted in solution at atmospheric pressure or in mechanochemical experiments under ball-mill grinding conditions, there is no necessity to use a base or thiolate anion as a catalyst for the exchange reaction under investigated conditions. Single-crystal and powder X-ray diffraction revealed also that, despite the high-pressure conditions of this reaction, the heterodimeric-disulfide product unexpectedly crystallizes into the low-density polymorph A. This counterintuitive result contrasts with the high-pressure stability of the higher-density polymorph B, confirmed by its compression up to 2.8 GPa with no signs of a phase transition.
sp3)C7-H7AC•••C3' 2.789(3) 2.726(2) 2.667(2) 2.668(6) x-1, y, z (sp2)C6-H6•••C5' 2.706(2) 2.661(2) 2.642(2) 2.632(5) x-0.5, y, 0.5-z
SPR vis-NIR spectroscopy of Au nanorods conveniently detects phase transitions and measures the refractive index under high pressure.
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