Raman and X-Ray Scattering Studies of High-Pressure Phases of Urea

• Published in: The journal of physical chemistry. B Vol. 109; no. 16; pp. 8206 - 8215
• Main Authors: Lamelas, F. J, Dreger, Z. A, Gupta, Y. M
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 28.04.2005
• Subjects: Pressure; Scattering, Radiation; Spectrum Analysis, Raman; Urea - chemistry; X-Rays
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp040760m

Single-crystal and polycrystalline urea samples were compressed to 12 GPa in a diamond-anvil cell. Raman-scattering measurements indicate a sequence of four structural phases occurring over this pressure range at room temperature. The transitions to the high-pressure phases take place at pressures near 0.5 GPa (phase I → II), 5.0 GPa (II → III), and 8.0 GPa (III → IV). Lattice parameters in phase I (tetragonal, with 2 molecules per unit cell, space group P4̄21 m ( )) and phase II (orthorhombic, 4 molecules per unit cell, space group P212121 (D 2 4)) were determined using angle-dispersive X-ray diffraction experiments. For phases III and IV, the combined Raman and diffraction data indicate that the unit cells are likely orthorhombic with four molecules per unit cell. Spatially resolved Raman measurements on single-crystal samples in phases III and IV reveal the coexistence of two domains with distinct spectral features. Physical origins of the spatial domains in phases III and IV are examined and discussed.