No abstract

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.

No abstract

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.

A sharp focus of current research on superconducting superhydrides is to raise their critical temperature Tc at moderate pressures. Here, we report a discovery of giant enhancement of Tc in CeH9 obtained via random substitution of half Ce by La, leading to equal-atomic (La,Ce)H9 alloy stabilized by maximum configurational entropy, containing the LaH9 unit that is unstable in pure compound form. The synthesized (La,Ce)H9 alloy exhibits Tc of 148–178 K in the pressure range of 97–172 GPa, representing up to 80% enhancement of Tc compared to pure CeH9 and showcasing the highest Tc at sub-megabar pressure among the known superhydrides. This work demonstrates substitutional alloying as a highly effective enabling tool for substantially enhancing Tc via atypical compositional modulation inside suitably selected host crystal. This optimal substitutional alloying approach opens a promising avenue for synthesis of high-entropy multinary superhydrides that may exhibit further increased Tc at even lower pressures.