The temperature dependence of the reflection intensities of the modulated composite structure Hg_0.776(BEDT‐TTF)SCN

The temperature dependence between 30 and 300 K of the intensities of 24 reflections of the column-composite structure Hg0.776(BEDT-TTF)SCN [Wang, Beno, Carlson, Thorup, Murray, Porter, Williams, Maly, Bu, Petricek, Cisarova, Coppens, Jung, Whangbo, Schirber & Overmyer (1991). Chem. Mater. 3, 508-513; BEDT-TTF = 3,4,3',4'-bis(ethylenedithio)-2,2',5,5'-tetrathiafulvalene] has been analyzed in terms of a model including phason temperature factors. The temperature dependence of the main and first-order satellite reflections is reasonably well reproduced in a refinement with 236 observations and four variables. The results are interpreted in terms of a temperature independence of the static displacement amplitudes. The room-temperature r.m.s. phason fluctuations of the mercury sublattice are 50 (2)degrees. This value implies that the mean mercury displacement amplitude will increase by approximately 60% on lowering of the temperature to within the liquid-helium range. The thermal contraction on cooling is the same for the two sublattices.