The effect of high pressure on the structural stability of oxamide has been investigated in a diamond anvil cell by Raman spectroscopy up to ∼14.6 GPa and by angle-dispersive X-ray diffraction (ADXRD) up to ∼17.5 GPa. The discontinuity in Raman shifts around 9.6 GPa indicates a pressure-induced structural phase transition. This phase transition is confirmed by the change of ADXRD spectra with the symmetry transformation from P1 to P1. On total release of pressure, the diffraction pattern returns to its initial state, implying this transition is reversible. We discuss the pressure-induced variations in N-H stretching vibrations and the amide modes in Raman spectra and propose that this phase transition is attributed to the distortions of the hydrogen-bonded networks.

Searching for nontoxic and stable perovskite-like alternatives to lead-based halide perovskites for photovoltaic application is one urgent issue in photoelectricity science. Such exploration inevitably requires an effective method to accurately control both the crystalline and electronic structures. This work applies high pressure to narrow the band gap of perovskite-like organometal halide, [NH-(CH)-NH]CuCl (DABCuCl), through the crystalline-structure tuning. The band gap keeps decreasing below ∼12 GPa, involving the shrinkage and distortion of CuCl. Inorganic distortion determines both band-gap narrowing and phase transition between 6.4 and 10.5 GPa, and organic chains function as the spring cushion, evidenced by the structural transition at ∼0.8 GPa. The supporting function of organic chains protects DABCuCl from phase transition and amorphization, which also contributes to the sustaining band-gap narrowing. This work combines crystal structure and macroscopic property together and offers new strategies for the further design and synthesis of hybrid perovskite-like alternatives.

In situ high-pressure Raman spectroscopy and synchrotron X-ray diffraction (XRD) have been employed to investigate the behavior of the energetic material urea nitrate ((NH2)2COH+·NO3 –, UN) up to the pressure of ∼26 GPa. UN exhibits the typical supramolecular structure with the uronium cation and nitrate anion held together by multiple hydrogen bonds in the layer. The irreversible phase transition in the range ∼9–15 GPa has been corroborated by experimental results and is proposed to stem from rearrangements of hydrogen bonds. Further analysis of XRD patterns indicates the new phase (phase II) has Pc symmetry. The retrieved sample is ∼10.6% smaller than the ambient phase (phase I) in volume owing to the transformation from two-dimensional (2D) hydrogen-bonded networks to three-dimensional (3D) ones. The mechanism for the phase transition involves the cooperativity of noncovalent interactions under high pressure and distortions of the layered structure. This work suggests high pressure is an efficient technique to explore the performance of energetic materials, and to synthesize new phases with high density.

The metastable wurtzite nanocrystals of CuGaS(2) have been synthesized through a facile and effective one-pot solvothermal approach. Through the Rietveld refinement on experimental X-ray diffraction patterns, we have unambiguously determined the structural parameters and the disordered nature of this wurtzite phase. The metastability of wurtzite structure with respect to the stable chalcopyrite structure was testified by a precise theoretical total energy calculation. Subsequent high-pressure experiments were performed to establish the isothermal phase stability of this wurtzite phase in the pressure range of 0-15.9 GPa, above which another disordered rock salt phase crystallized and remained stable up to 30.3 GPa, the highest pressure studied. Upon release of pressure, the sample was irreversible and intriguingly converted into the energetically more favorable and ordered chalcopyrite structure as revealed by the synchrotron X-ray diffraction and the high-resolution transmission electron microscopic measurements. The observed phase transitions were rationalized by first-principles calculations. The current research surely establishes a novel phase transition sequence of disorder → disorder → order, where pressure has played a significant role in effectively tuning stabilities of these different phases.

We report the high-pressure response of guanidinium methanesulfonate (C(NH(2))(3)(+)·CH(3)SO(3)(-), GMS) using in situ Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques up to the pressures of ~11 GPa. GMS exhibits the representative supramolecular structure of two-dimensional (2D) hydrogen-bonded bilayered motifs under ambient conditions. On the basis of the experimental results, two phase transitions were identified at 0.6 and 1.5 GPa, respectively. The first phase transition, which shows the reconstructive feature, is ascribed to the rearrangements of hydrogen-bonded networks, resulting in the symmetry transformation from C2/m to Pnma. The second one proves to be associated with local distortions of methyl groups, accompanied by the symmetry transformation from Pnma to Pna2(1). The cooperativity of hydrogen bonding, electrostatic, and van der Waals interactions, as well as mechanisms for the phase transitions is discussed by means of the local nature of the structure.

We report the high-pressure response of three forms (α, δ, and γ) of pyrazinamide (C(5)H(5)N(3)O, PZA) by in situ Raman spectroscopy and synchrotron X-ray diffraction techniques with a pressure of about 14 GPa. These different forms are characterized by various intermolecular bonding schemes. High-pressure experimental results show that the γ phase undergoes phase transition to the β phase at a pressure of about 4 GPa, whereas the other two forms retain their original structures at a high pressure. We propose that the stabilities of the α and δ forms upon compression are due to the special dimer connection that these forms possess. On the other hand, the γ form, which does not have this connection, prefers to transform to the closely related β form when pressure is applied. The detailed mechanism of the phase transition together with the stability of the three polymorphs is discussed by taking molecular stacking into account.

High-pressure behaviors of hydrogen-bonded molecular crystal, sulfamic acid (NH3(+)SO3(-), SA), have been investigated using Raman spectroscopy and synchrotron X-ray diffraction (XRD) techniques up to the pressure of ~20 GPa. Under ambient conditions, molecules of SA are arranged in puckered layers and held together by hydrogen bonding and electrostatic interactions. It is proved by the Raman results that SA undergoes the molecular conformation changes in the pressure range 8.1-10.2 GPa. Then between 10.2 and 12.7 GPa, a phase transition is observed in both Raman and XRD patterns. Both the ambient and high-pressure phases of SA crystallize in Pbca symmetry with similar unit-cell dimensions. The mechanism of the phase transition involves relative movements of adjacent hydrogen-bonded molecules, accompanied by the rearrangement of hydrogen bonds and the enhancement of electrostatic interactions.

We report the results of high-pressure Raman and X-ray diffraction measurements performed on ammonium squarate ((NH(4))(2)C(4)O(4), AS), a representative supramolecular architecture based on hydrogen bonding and π-stacking interactions, at various pressures up to 19 GPa. Two phase transitions at ∼2.7 GPa and in the pressure range of 11.1-13.6 GPa were observed. Both Raman and XRD results provide convincing evidence for these two phase transitions. The first phase transition is attributed to the rearrangements of hydrogen-bonding networks, resulting in the symmetry transformation from P2(1)/c to P1. The second one, which is identified as an order-disorder phase transition, arises from significant modifications of squarate rings and random orientations of NH(4)(+) cations. The cooperative effects between hydrogen-bonding and π-stacking interactions, as well as mechanisms for the phase transitions, are discussed by virtue of the local structure of AS.