Analogy of the slow dynamics between the supercooled liquid and the supercooled plastic crystal states of difluorotetrachloroethane

Slow dynamics of difluorotetrachloroethane in both supercooled plastic crystal and supercooled liquid states have been investigated from Molecular Dynamics simulations. The temperature and wave-vector dependence of collective dynamics in both states are probed using coherent dynamical scattering functions $S(Q,t)$. Our results confirm the strong analogy between molecular liquids and plastic crystals for which $\alpha$-relaxation times and non-ergodicity parameters are controlled by the non trivial static correlations $S(Q)$ as predicted by the Mode Coupling Theory. The use of infinitely thin needles distributed on a lattice as model of plastic crystals is discussed

Glassy behavior of molecular crystals: A comparison between results from MD-simulation and mode coupling theory

We have investigated the glassy behavior of a molecular crystal built up with chloroadamantane molecules. For a simple model of this molecule and a rigid fcc lattice a MD simulation was performed from which we obtained the dynamical orientational correlators $S_{\lambda \lambda '}({\bf{q}},t)$ and the ``self'' correlators $S_{\lambda \lambda '}^{(s)}(t)$, with $\lambda = (\ell, m)$, $\lambda' = (\ell', m')$. Our investigations are for the diagonal correlators $\lambda = \lambda'$. Since the lattice constant decreases with decreasing temperature which leads to an increase of the steric hindrance of the molecules, we find a strong slowing down of the relaxation. It has a high sensitivity on $\lambda$, $\lambda '$. For most $(\ell,m)$, there is a two-step relaxation process, but practically not for $(\ell,m) = (2,1)$, $(3,2)$, $(4,1)$ and $(4,3)$. Our results are consistent with the $\alpha$-relaxation scaling laws predicted by mode coupling theory from which we deduce the glass transition temperature $T_c^{MD} \cong 217K$. From a first principle solution of the mode coupling equations we find $T_c^{MCT} \cong 267K$. Furthermore mode coupling theory reproduces the absence of a two-step relaxation process for $(\ell,m)=(2,1)$, $(3,2)$, $(4,1)$ and $(4,3)$, but underestimates the critical nonergodicity parameters by about 50 per cent for all other $(\ell,m)$. It is suggested that this underestimation originates from the anisotropic crystal field which is not accounted for by mode coupling theory. Our results also imply that phonons have no essential influence on the long time relaxation

Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals

By means of NMR experiment and MD computer simulation we investigate the dynamical properties of a chloroadamantane orientationally disordered crystal. We find a plastic-plastic dynamical transition at T_x ~ 330 K in the pico-nanosecond regime. It is interpreted as the rotational analogue of the Goldstein crossing temperature between quasi-free diffusion and activated regime predicted in liquids. Below T_x, NMR experimental data are well described by a Frenkel model corresponding to a strongly anisotropic motion. At higher temperatures, a drastic deviation is observed toward quasi-isotropic rotational diffusion. Close to T_x, we observe that two-step relaxations emerge. An interpretation which is based on the present study of a specific heat anomaly detected by a recent calorimetric experiment is proposed.Comment: 4 pages, 4 figures; changed abstract and references; corrected figure

Onset of slow dynamics in difluorotetrachloroethane glassy crystal

Complementary Neutron Spin Echo and X-ray experiments and Molecular Dynamics simulations have been performed on difluorotetrachloroethane (CFCl2-CFCl2) glassy crystal. Static, single-molecule reorientational dynamics and collective dynamics properties are investigated. The orientational disorder is characterized at different temperatures and a change in nature of rotational dynamics is observed. We show that dynamics can be described by some scaling predictions of the Mode Coupling Theory (MCT) and a critical temperature $T_{c}$ is determined. Our results also confirm the strong analogy between molecular liquids and plastic crystals for which $\alpha$-relaxation times and non-ergodicity parameters are controlled by the non trivial static correlations as predicted by MCT

Is there something of the MCT in orientationally disordered crystals ?

Molecular Dynamics simulations have been performed on the orientationally disordered crystal chloroadamantane: a model system where dynamics are almost completely controlled by rotations. A critical temperature T_c = 225 K as predicted by the Mode Coupling Theory can be clearly determined both in the alpha and beta dynamical regimes. This investigation also shows the existence of a second remarkable dynamical crossover at the temperature T_x > T_c consistent with a previous NMR and MD study [1]. This allows us to confirm clearly the existence of a 'landscape-influenced' regime occurring in the temperature range [T_c-T_x] as recently proposed [2,3].Comment: 4 pages, 5 figures, REVTEX

Microscopic theory of glassy dynamics and glass transition for molecular crystals

We derive a microscopic equation of motion for the dynamical orientational correlators of molecular crystals. Our approach is based upon mode coupling theory. Compared to liquids we find four main differences: (i) the memory kernel contains Umklapp processes, (ii) besides the static two-molecule orientational correlators one also needs the static one-molecule orientational density as an input, where the latter is nontrivial, (iii) the static orientational current density correlator does contribute an anisotropic, inertia-independent part to the memory kernel, (iv) if the molecules are assumed to be fixed on a rigid lattice, the tensorial orientational correlators and the memory kernel have vanishing l,l'=0 components. The resulting mode coupling equations are solved for hard ellipsoids of revolution on a rigid sc-lattice. Using the static orientational correlators from Percus-Yevick theory we find an ideal glass transition generated due to precursors of orientational order which depend on X and p, the aspect ratio and packing fraction of the ellipsoids. The glass formation of oblate ellipsoids is enhanced compared to that for prolate ones. For oblate ellipsoids with X <~ 0.7 and prolate ellipsoids with X >~ 4, the critical diagonal nonergodicity parameters in reciprocal space exhibit more or less sharp maxima at the zone center with very small values elsewhere, while for prolate ellipsoids with 2 <~ X <~ 2.5 we have maxima at the zone edge. The off-diagonal nonergodicity parameters are not restricted to positive values and show similar behavior. For 0.7 <~ X <~ 2, no glass transition is found. In the glass phase, the nonergodicity parameters show a pronounced q-dependence.Comment: 17 pages, 12 figures, accepted at Phys. Rev. E. v4 is almost identical to the final paper version. It includes, compared to former versions v2/v3, no new physical content, but only some corrected formulas in the appendices and corrected typos in text. In comparison to version v1, in v2-v4 some new results have been included and text has been change

Structure and relaxations in liquid and amorphous Selenium

We report a molecular dynamics simulation of selenium, described by a three-body interaction. The temperatures T_g and T_c and the structural properties are in agreement with experiment. The mean nearest neighbor coordination number is 2.1. A small pre-peak at about 1 AA^-1 can be explained in terms of void correlations. In the intermediate self-scattering function, i.e. the density fluctuation correlation, classical behavior, alpha- and beta-regimes, is found. We also observe the plateau in the beta-regime below T_g. In a second step, we investigated the heterogeneous and/or homogeneous behavior of the relaxations. At both short and long times the relaxations are homogeneous (or weakly heterogeneous). In the intermediate time scale, lowering the temperature increases the heterogeneity. We connect these different domains to the vibrational (ballistic), beta- and alpha-regimes. We have also shown that the increase in heterogeneity can be understood in terms of relaxations