Determine the crystal structure and transition mechanism of phase X

Determine the crystal structure of phase X in compressed urea and establish the nature of its transformation into the centrosymmetric phase V′, which remains unresolved because the phase-X crystal structure has not been solved.

Background

Under room-temperature compression, urea exhibits an intermediate phase X between phase III and the high-pressure centrosymmetric phase V′. Powder diffraction tentatively indexes phase X to an orthorhombic lattice with space group Pnnm, but the authors emphasize that this assignment is not definitive because of the phase’s disordered character.

The diffraction patterns and lattice-phonon behavior change continuously from phase X to phase V′, while the observed hysteresis is consistent with a first-order transition. The authors consider whether this transition is displacive, but a definitive interpretation depends on obtaining a reliable crystal-structure solution for phase X.