2D displacement pattern in TSeF-TCNQ model analysis of the 2k F diffuse lines

2014 The observed intensity distribution of the X-ray 2 kF diffuse lines of TSeF-TCNQ has been reproduced by a detailed model of rigid molecular translations with components along the band c-axes, including weak correlations between translations on the neighbouring chains in the c-direction. Due to overwhelmingly larger molecular scattering factor, only TSeF was taken into account. Good semiquantitative agreement, including detailed substructures over broad maxima, was obtained between the calculated curves and observed data from photographs for three different crystal orientations. In the movement of the 2 kF soft mode, the two-dimensional (2D) displacement pattern in each TSeF sheet is close to that in which each molecule moves nearly parallel to its molecular plane. The c-components are in phase and the b-components are antiphase on the nearest neighbour chains, following the alternate change of the tilt direction of the molecular planes from chain to chain. A small difference from this simple picture is featured by a phase progression of about 65° per unit cell in the c-direction. No features were discernible as coming from libration modes. J. Physique 42 (1981) 1327-1343 SEPTEMBRE 1981,