Exotic centrosymmetric phase of acentric urea under high pressure
Urea is a simple prototype supramolecular crystal that exhibits rich polymorphism at low pressure due to broken and restored N-H-O hydrogen bonds. The high pressure polymorph (phase V') of acentric urea crystallizes in a centrosymmetric structure, which presents an appealing target because of its potential exotic structure, analogous to the symmetric ice phase X. The pressure-induced polymorphism of urea was studied using powder X-ray diffraction, infrared and Raman spectroscopy, second harmonic generation (SHG) measurements up to 20 GPa and ab initio crystal structure prediction (CSP) based on the constrained evolutionary approach. A strong decrease of the SHG signal at the transition pressure 10 GPa reveals that the high-pressure polymorph is indeed centrosymmetric, further confirmed by the selection rules observed in the lattice vibration modes, in contrast to chemical intuition for acentric urea. The structural evolution sequence obtained from X-ray diffraction, SHG and CSP calculations is as follows: phase I (P421m; Z=2) from 0 to 0.5 GPa, Phase III (P212121; Z=4) from 0.5 to 5.2 GPa, and phase V' (P21/m; Z=6) beyond 10.0 GPa which is energetically competitive with the theoretically predicted phase V (Pnma; Z=4). A phase X with distinct spectral and diffraction features forms between 5.2 and 10.0 GPa, which could be explained by a quantum disorder intermediate state between phase III and V', that is ascribed to the difficulty to disrupt the H-bonding network under extremely compressed environment. The softening of N-H vibrations and the change in intensity of the vibrations associated with the hydrogen bonding provide evidence for proton tunneling and charge-transfer interaction in phase X.