Rock salt-urea-water (1/1/1) at 293 and 117 K

The crystal structure of NaCl.CH(4)N(2)O.H(2)O has been determined at 117 K and redetermined at room temperature. It can be described as consisting of alternating ;organic' and ;inorganic' planar layers. While at room temperature the structure belongs to the space group I2, the low-temperature structure belongs to the space group Pn2(1)m. All water O atoms are located on positions with crystallographic symmetry 2 (m) in the room-temperature (low-temperature) structure, which means that the water molecules belong, in both cases, to point group mm2. During the phase transition, half of the urea molecules per unit cell perform a 90 degrees rotation about their respective C-O axes. The other half and the inorganic parts of the structure remain unaltered. The relationship between the two phases is remarkable, inasmuch as no obvious reason for the transition to occur could be found; the internal structures of all components of the two phases remain unaltered and even the interactions between the different parts seem to be the same before and after the transition (at least when looked at from an energetic point of view).