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Intermolecular interactions and aggregation in compressed N,N-dimethylformamide (DMF, C3H7NO, mp 212.7 K) have been determined in the solid state by single-crystal X-ray diffraction. The crystals of DMF were obtained under high-pressure isochoric conditions (the structure was determined up to 1.7 GPa) and under low-temperature isobaric conditions (the structure was determined down to 100 K). The exceptional stability of the DMF structure has been connected with its large number of degrees of freedom, due to the low triclinic symmetry of the crystal and two symmetry-independent molecules. Consequently, the distortions of different CH···O bonds linking the molecules into one-dimensional aggregates, and their mutual displacements, efficiently compensate the structural strains generated by pressure.

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, MHy2PbBr4, 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 MHy2PbBr4undergoes an unusual Pmn21→ P21phase 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–NH2interactions, an unprecedented threefold increase of positive linear compressibility perpendicular to the polyanionic layers and a large negative linear compressibility of −22.39 TPa–1within 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 Pmn21phase show the band gap narrowing, followed by its widening in the high-pressure P21phase. 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 PbBr6octahedra 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.