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 P42(1)m (D3(2d))) and phase II (orthorhombic, 4 molecules per unit cell, space group P2(1)2(1)2(1) (D2(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.

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Neutron-scattering studies of a single crystal of sodium have been carried out at large and small scattering angles. Near the sodium 110 Bragg reflection, we find features which are similar to those which have been observed previously in potassium. Using additional data gathered at small scattering angles, we show that the features near 110 can be quantitatively assigned to an experimental double-scattering artifact, in terms of peak intensities as well as positions. Overall, we find no neutron-scattering evidence of charge-density waves in sodium.