| As one of the well-known basic thermodynamic parameters, pressure can be effective in tuning the interatomic distances, potentials, and crystal structures. Thus, pressure is a powerful tool to explore the properties of materials and design new materials which exhibit excellent performance. Diamond anvil cell technique, combined with synchrotron X-ray diffraction, IR and Raman spectroscopies, have been widely used in geoscience, material science, chemistry, and physics, which is of fundamental importance not only for scientific research(e.g., symmetrization of hydrogen bond,molecular polymerization), but also for potential practical applications(e.g., polymorphism of drugs, crystal engineering). Molecular crystals are composed of molecules held together by intermolecular interactions,such as hydrogen bond, van der Waals force, and electrostatic interactions. In particular, hydrogen bond is the most important and extensively investigated intermolecular interaction, because it widely exists in inorganic materials, organic materials, and biological systems, such as water and amino acids. Compared with covalent bonds,hydrogen bonds are far weaker, so geometric parameters of hydrogen bonds(e.g., bond strength, bond length, bond angle) can be altered more easily by external forces. Large modifications can be observed in hydrogen bonds, including breakage, formation, distortion, rearrangement, and symmetrization. In hydrogen-bonded organic molecules, the changes in hydrogen bond can lead to variations of crystal structures, which has effects on the properties of materials, such as compressibility, electric conductivity, and thermal conductivity. In this article, the high pressure behaviors of some typical hydrogen-bonded organic crystals are investigated. The behaviors of hydrogen bond, crystal structures and compressibility, as well as the mechanism of phase transitions are studied using in situ high pressure Raman spectroscopy, synchrotron X-ray diffraction and the first-principle calculations. And the obtained results are as follows:(1) Phase transition of ammonium formate(AF) under high pressure. AF is one of the simplest three-dimensional hydrogen-bonded structures. AF is widely used in scientific research and pharmaceuticals industry and has attracted much attention due to its important applications and unique properties. The high pressure behaviors of AF are investigated by in situ high pressure Raman scattering, synchrotron X-ray diffraction, and the first-principle calculations up to 20 GPa. A phase transition can be identified at 1.8 GPa, indicated by the obvious changes in Raman spectra, some new peaks appear while the original peaks disappear. Two new N-H stretching modes appear, which indicates the formation of new hydrogen bonds in AF. Meanwhile, the pressure dependence of Raman modes also show discontinuities at 1.8 GPa. All the abrupt changes in Raman spectra indicate that a phase transition occurs at 1.8 GPa. In order to obtain the structural information of the high pressure phase, in situ high pressure synchrotron X-ray diffraction(XRD) is also performed. A new set of diffraction patterns appears at 1.8 GPa, indicating the structural change of AF. Rietveld refinement of the XRD pattern at 1.8 GPa is carried out, the high pressure phase belongs to monoclinic system with crystal symmetry of P21. Axial compressibility of the ambient phase shows obvious anisotropy, the a and c axes are more compressible than b axis because of the compact ions along the b direction. Furthermore, there is a large contraction of unit cell volume across the phase transition(～12%), indicating that the high pressure phase is a more compact structure. The first-principle calculations is performed to gain more insight into the mechanism of the phase transition. The phase transition is proposed to be the rearrangement of hydrogen bonding networks. The inferred mechanism is consistent with features observed in the Raman and XRD results. Upon total release of pressure, the diffraction pattern returns to its initial state, indicating the phase transition is reversible.(2) High pressure induced phase transitions in acetamide. Acetamide is a representative compound for the amide linkage in peptides and proteins with a simple hydrogen-bonded structure. The high pressure behavior of acetamide is investigated by in situ high pressure Raman scattering, synchrotron X-ray diffraction, and the first-principle calculations up to 10 GPa. Two phase transitions are observed at 0.9 and 3.2 GPa by the obvious changes in the Raman spectra as well as the discontinuities of peak positions versus pressure. Meanwhile, rearrangements of hydrogen bonds can be concluded from the redistribution of intensities and positions of N-H vibrations across the phase transitions. Because Raman spectra cannot provide structural information of the high pressure phases, high-pressure X-ray diffraction is performed. New diffraction peaks can be observed at 1.0 GPa and 3.5 GPa, which further confirm the occurrence of the phase transitions. We performed Pawley refinement of the diffraction pattern at 2.2 GPa. The first high pressure phase belongs to monoclinic system with a possible C2/c space group. The high pressure structure has a lower symmetry, which is consistent with the splitting of internal mode in Raman spectra. Based on the Raman and XRD results, we proposed the mechanism of the phase transitions. High pressure induces the rearrangements of hydrogen bonds, and the changes of the crystal structure. The phase transitions are reversible. After decompression, the high pressure phases transforms to the ambient structure.(3) High pressure structural investigation of benzoic acid. As the simplest aromatic carboxylic acid, benzoic acid(BA) can represent a model system of many active pharmaceutical ingredients(e.g., aspirin, salicylic acid, flufenamic acid, diflunisal). At ambient conditions, each two monomers are linked by two intermolecular O-H···O hydrogen bonds. By using in situ high pressure Raman scattering, synchrotron X-ray diffraction, and the first-principle calculations, the axial compressibility, bulk modulus, and the behaviors of hydrogen bond are studied. Small changes(e.g., emergence of new peaks, splitting of original peaks) can be observed in the Raman spectra at high pressures. However, no obvious changes can be observed in the X-ray diffraction measurements, which rules out any symmetry/structure changes within this pressure range. Axial compressibility shows obvious anisotropy, the a axis is more compressible than b and c axes, because BA molecules form more compact structure along b and c axes. Hydrogen bond is stronger than van der Waals interaction, thus showing that it is much harder to be compressed along b and c direction. The unit cell volume is reduced by about 32.4% in the pressure range of 0-18.1 GPa, showing considerable compressibility, which is associated with the weak intermolecular interactions. We performed the first-principle calculations to understand the behaviors of hydrogen bond. The results show that the bond length of hydrogen bond(O-H···O) reduced with increasing pressure. Meanwhile, the bond length of O-H increased while the bond distance of H···O reduced, and they show almost equal bond lengths at about 20 GPa. The close bond length between O-H and H···O indicates the electron cloud of the H atom is almost equally distributed between the two O atoms, and the electrostatic attraction between H and O atoms(H···O) possess a substantial covalent characteristic. Thus, the symmetrization of hydrogen bond is expected in BA at higher pressures. |
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