The structural stability of benzoic acid (C 6 H 5 COOH, BA), a hydrogen-bonded molecular crystal, has been investigated by Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) up to ~ 18 GPa at room temperature. Under ambient conditions, benzoic acid molecules are arranged in two set of parallel planes and held together by hydrogen bonding and van der Waals interactions. 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. The pressure dependence of lattice parameters are presented, which show monotonously decrease without any anomalies. The experimental isothermal pressure-volume data are well fitted by the third-order Birch-Murnaghan equation of state, yielding bulk modulus 0 B = 41.7(6) GPa and a first pressure derivative ' 0 B = 4.5(4). Axial compressibility shows obvious anisotropy, the a-axis is more compressible than b-axis and c-axis. Moreover, the near symmetrization limit of hydrogen bonds at high pressures is proposed from the first-principles calculations.Based on the Raman, XRD, and the first-principles calculations analysis, we propose that the high pressure structural stability of benzoic acid is associated with the special hydrogen-bonded dimer structure.

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High-pressure behaviors of hydrogen-bonded supramolecular structure, ammonium formate (NH4 +COOH–, AF), have been investigated under pressure by in situ synchrotron X-ray diffraction (XRD) and Raman spectroscopy up to 20 GPa. Under ambient conditions, AF exhibits three-dimensional hydrogen-bonded networks with two molecules crystallize in a monoclinic unit cell of space group Pc. A structural phase transition can be identified at around 1.8 GPa, as indicated by the abrupt changes in Raman spectra as well as the pressure dependence of major Raman modes. Furthermore, two new N–H stretching modes emerge, indicative of the construction of new hydrogen bonds. Rearrangement of the hydrogen-bonded networks is also deduced by the obvious changes of N–H stretching modes both in position and intensity. The reversible phase transition is confirmed by in situ synchrotron XRD experiments with the emergence of a new set of diffraction pattern. The high-pressure phase is found to have a structure with a monoclinic unit cell (space group P21) containing two molecules. The structural transformation is proposed to be a result of the rearrangement of the hydrogen-bonded networks. Detailed mechanism for the phase transition, high-pressure behaviors of hydrogen bonds, as well as the cooperativity of different noncovalent interactions are presented and discussed.

The structural and vibrational properties of acetamide under high pressure were probed by in-situ synchrotron X-ray diffraction (XRD) and Raman scattering up to ~ 10 GPa. Two structural phase transitions are observed at 0.9 and 3.2 GPa, evidenced by the obvious changes in Raman spectra as well as the discontinuities of peak positions versus pressure. The phase transitions are further confirmed by the significant changes of XRD patterns. The two phase transitions are proposed to originate from the rearrangements of hydrogen-bonded networks, deduced by the redistribution of intensities and positions of N-H vibrations. Detailed analysis of XRD patterns indicates that the first high-pressure phase (phase II) possesses a monoclinic structure with a possible space group of C2/c. Moreover, the phase transitions are reversible since the diffraction pattern returns to its initial state upon total decompression. The detailed mechanisms for these phase transitions, the cooperativity of different intermolecular interactions, as well as the high-pressure behaviors of hydrogen bonds are presented and discussed.

High-pressure behaviors of ammonium perchlorate (NH4ClO4), a widely used energetic oxidizer, have been investigated using in situ angle-dispersive X-ray diffraction (XRD) and Raman spectroscopy experiments up to 22.0 and 20.4 GPa, respectively. A sluggish structural transformation is identified, which started at ∼2.4 GPa and completed at ∼10.6 GPa. The crystal structure of high pressure (phase II) is refined to possess a symmetry of P2. The structural transformation is indicated by the obvious changes in XRD patterns, Raman spectra, and discontinuities of peak positions. Variations in lattice constants and volume of unit cell of the ambient structure (phase I) up to ∼10.0 GPa are also presented. The compressibility of phase I is anisotropic. The compression ratio of b axis is larger than that of a and c axes. The bulk modulus of phase I is B 0 = 24.5(4) GPa, and its first pressure derivative is B 0′ = 4.5(2). The phase transition is reversible, as the XRD pattern transformed to the initial profile upon release of pressure. On the basis of the Raman and XRD analysis, the high-pressure-induced phase transition of ammonium perchlorate is proposed to be associated with rearrangement of hydrogen-bonded networks.