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Abstract
The incommensurate structure of BCCD at 130 K with q = 0.296c* has been determined in the superspace-group P(Pnma):(1, s, −1) by means of single crystal X-ray diffraction data including main and first-order satellite reflections. Only zero and first-order harmonics have been considered in the modulation functions. The final agreement factors are R = 0.048, Ro = 0.042 and R1 =0.067, for all, main and first-order satellite reflections respectively. Only the atomic modulations of the betaine molecule could be interpreted in terms of rigid-body modulated motions. The symmetry of the structure is in accordance with a single A3 soft-mode model for the phase transition sequence.
