High-Pressure Transformations and the Resonance Structure of Thiourea

High pressure increases intermolecular interactions in the crystal environment and affects the molecular dimensions of thiourea, CS(NH2)(2), consistently with changing contributions of the thione and zwitterionic mesomers. The ambient-pressure phase V (space-group Pnma, Z = 4) at 0.34 GPa transforms to phase VI, of the same space group symmetry, but with the larger unit-cell trifold (Z = 12). Another pressure-induced transition to phase VII was previously postulated for thiourea at 0.54 GPa; however, no associated structural transformations were identified. Our high-pressure single-crystal X-ray diffraction study reveals that following the transition at 0.34 GPa the lattice parameters and crystal structure strongly and monotonically change to about 0.65 GPa while the crystal symmetry is retained.