Modulated structure of thiourea

The modulated structure of SC(NH2)2 has been determined using the superspace-group formalism, in the superspace group P(Pnma):(santi 11). Data include main reflections and up to third-order satellite reflections. Three harmonics have been included in the modulation. The superspace description of the modulation is discussed in detail and compared with that using symmetry modes. The final agreement factors are R=0.046, R0=0.038, R1=0.046, R2=0.089 and R3=0.115, for all reflections, main reflections, and first-, second- and third-order satellites, respectively. The structure was investigated in the commensurate phase with modulation wavevector q=1/9b*, but the analysis was performed considering the modulation as incommensurate. The results essentially agree with those recently obtained by Tanisaki and Mashiyama, using a standard commensurate approach for a ninefold structure. Other structural models proposed recently were checked with negative results. The atomic modulations could be interpreted in terms of rigid-body modulated motions. The anharmonicity of the modulation functions is considerable, with non-negligible contributions of second and third harmonics. The form of the rigid-body modulation functions indicates an incipient soliton regime. Crystal data of the average structure: Mr=76.07, orthorhombic, Pnma, a=7.5429 (8), b=8.5422 (7), c=5.4647 (4) A, V=352.10 (5) A3, Z=4, Dx=1.44 g cm-3, λ(CuKα)=1.5418 A, μ=59.1 cm-1, F(000)=160, T=168 K, wavevector q=0.111 b*. (orig.)