Zakład Fizyki Wysokich Ciśnień - publikacje

Abstract: The results of a detailed DSC, X-ray powder diffraction and NMR investigation of n-decylammonium chloride (C10H21, NH3CI) are reported. If a sample in the interdigitated I phase is heated, it transforms to the non-interdigitated phase at ~ 328 K and then to the β and α phase at ~ 334 K. On cooling the transitions to the β and α phases occur at ~ 322 K and 319 K, respectively, but instead of returning to the I phase, the sample transforms to the ε phase at 297 K. The ε phase is metastable, but becomes stable below ~ 210 K. If the sample is heated from a temperature below 210 K, it shows a transition to the ε’ phase at ~ 288 K and then to the δ, β and α phases. A sample heated from the metastable ε phase does not show the transition to the ε’ phase, but exhibits an exothermic transition at ~ 274 K. In the e phase the intensity of diffraction peaks are relatively weak due to a disordered lattice. If the temperature is lowered from the δ → ε transition temperature the relatively large expansion coefficient of the a cell dimension above 210K is attributed to the gradual freezing of chain reorientations. Below 210 K the expansion coefficients are similar to those of the I phase. If the sample is now heated to temperatures above 210K, the temperature dependence of the length of the c axis becomes anomalous. In the I phase the methyl groups execute classical threefold reorientations, while NH3 groups jump among three potential wells with unequal depths. Chain reorientations only start to influence the results in the vicinity of the transition to the high temperature phases. On cooling from the high temperature phases the chains execute fourfold reorientations about their long axes, which are reduced to twofold reorientations at lower temperatures. Below 210 K all chain motions are frozen. If the sample is heated above 210 K the chains are limited to twofold reorientations which are frozen within a few hours. The results suggest that the potential barriers to chain reorientations are a function of time. © 1992 Taylor & Francis Group, LLC.