Pressure-Induced Phase Transition in N–H···O Hydrogen-Bonded Molecular Crystal Biurea: Combined Raman Scattering and X‑ray Diffraction Study

Abstract

The response of biurea to high pressures is investigated by <i>in situ</i> Raman spectroscopy and angle-dispersive X-ray diffraction (ADXRD) in a diamond anvil cell up to ∼5 GPa. Raman scattering measurements indicate a phase transition occurring over the pressure range of 0.6–1.5 GPa. Phase transition is confirmed by changes in the ADXRD spectra with symmetry transformation from <i>C</i>2/<i>c</i> to a possible space group <i>P</i>2/<i>n</i>. Upon total release of pressure, the diffraction spectrum returns to its initial state, which implies that the transition observed is reversible. We discuss variations in the Raman spectra, including splitting of modes, appearance of new modes, and abrupt changes in the slope of the frequency shift curves at several pressures. We propose that the phase transition observed in this study is attributed to rearrangement of the hydrogen-bonded networks