Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals

Affouard, Frédéric; Cochin, E.; Decressain, R.; Descamps, Marc

doi:10.1209/epl/i2001-00196-9

Cited by 21 publications

(39 citation statements)

References 27 publications

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“…Another one is chloroadamantane, which exhibits an equilibrium phase transition at T exp eq ffi 244 K [21]. A simple model for chloroadamantane (see next section) has been studied by MD-simulations [22,23]. A MCT analysis for the critical amplitudes and the a-relaxation time have demonstrated consistency with MCT predictions [23].…”

Section: Introductionmentioning

confidence: 88%

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

Ricker

Affouard²,

Schilling

et al. 2006

Journal of Non-Crystalline Solids

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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 kk 0 ðq; tÞ and the 'self' correlators S ðsÞ kk 0 ðtÞ, with k = (', m), k 0 = (' 0 , m 0 ). Our investigations are for the diagonal correlators k = k 0 . 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 k, k 0 . For most (', m), there is a two-step relaxation process, but practically not for (', m) = (2, 1), (3, 2), (4, 1) and (4, 3). Our results are consistent with the a-relaxation scaling laws predicted by mode coupling theory from which we deduce the glass transition temperature T MD c ffi 217 K. From a first-principle solution of the mode coupling equations we find T MCT c ffi 267 K. Furthermore mode coupling theory reproduces the absence of a two-step relaxation process for (', m) = (2, 1), (3, 2), (4, 1) and (4, 3), but underestimates the critical nonergodicity parameters by about 50 per cent for all other (', 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.

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Section: Introductionmentioning

confidence: 88%

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

Ricker

Affouard²,

Schilling

et al. 2006

Journal of Non-Crystalline Solids

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“…31 Both functions can also be related to the informations obtained from NMR relaxation measurements. 21 In order to investigate collective dynamics, the intermediate scattering function as it can be classically obtained from coherent neutron-scattering experiments will be mainly considered:…”

Section: Experiments and Details Of The Simulationmentioning

confidence: 99%

Onset of slow dynamics in difluorotetrachloroethane glassy crystal

Affouard

Cochin²,

Danède³

et al. 2005

The Journal of Chemical Physics

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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. Our results confirm the strong analogy between molecular liquids and plastic crystals. The orientational disorder is characterized at different temperatures and a change in the nature of rotational dynamics is observed. A careful check of the rotational diffusion model is performed using self-angular correlation functions Cl with high l values and compared to results obtained on molecular liquids composed of A-B dumbbells. Below the crossover temperature at which slow dynamics emerge, we show that some scaling predictions of the mode coupling theory hold and that alpha-relaxation times and nonergodicity parameters are controlled by the nontrivial static correlations.

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“…Considering the size of the abnormal heat capacity and the orientational disorder of the plastic phase, this jump does not seem to be related to a Schottky anomaly, as suggested by Kobashi et al, but rather to a change in the orientational arrangement and/or in its underlying dynamics. ClA, together with some other adamantane derivatives, has been the subject of several studies by means of broadband dielectric spectroscopy (BDS) [3,[21][22][23][24], nuclear magnetic resonance (NMR) [25,26], differential thermal analysis (DTA) [24,27], x-ray scattering [19,[28][29][30][31], IR experiments [32], calorimetry [17], quasielastic neutron scattering (QENS) [33,34] and molecular dynamics (MD) simulations [35,36].…”

mentioning

confidence: 99%

“…The only exception are the works of Affouard et al where an indication of a change in the dynamics at a temperature close to the specific-heat anomaly at 320 K was found via the concurrent use of MD, NMR, and Raman scattering [35,36]. However, only a vague explanation was found in those works that ascribed the calorimetric hum to a "local orientational ordering phenomena".…”

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confidence: 99%

On the microscopic mechanism behind the purely orientational disorder–disorder transition in the plastic phase of 1-chloroadamantane

Vispa

Monserrat

Cuello

et al. 2017

Phys. Chem. Chem. Phys.

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Globular molecules of 1-Chloroadamantane form a plastic phase in which the molecules rotate in a restrained way, but being their centers of mass forming a crystalline ordered lattice. Plastic phases can be regarded as test cases for the study of disordered phases since, contrary to what happens in the liquid phase, there is a lack of stochastic translational degrees of freedom. When the temperature is increased a hump in the specific heat is observed indicating a change in the energetic footprint of the dynamics of the molecules. This change takes place without a change in the symmetry of the crystalline lattice, i.e. no first-order transition is observed between temperatures below and above the calorimetric hump. This implies that subtle changes in the dynamics of the disordered plastic phase concerning purely orientational degrees of freedom should appear at the thermodynamic anomaly. Accordingly, we describe, for the first time, the microscopic mechanisms behind a disorder-disorder transition through the analysis of neutron diffraction and QENS experiments. Results evince a change at the molecular rotational dynamics accompanied by a continuous change in density.PACS numbers: 33.15. Fm,61.20.Ja, Molecular disordered systems, such as liquid and plastic phases, are still far to be completely understood. Even simple systems with very restricted disorder, as those where atoms have a fractional occupancy of crystallographic sites, are still a matter of controversy [1][2][3][4]. In order to gain some insights about the role of disorder into the properties of matter, the study of systems with constrained disorder is very relevant. In the case of our work, we have studied a plastic phase for which only the orientational disorder is relevant, being the positional order restrained since the molecular centers-ofmass occupy a well-defined lattice position. This fact makes these systems very interesting models for canonical liquids. Specifically, they are useful to investigate the role of translational and orientational degrees of freedom in disordered phases. For liquids this is accounted in the so-called two-order parameter description of liquids [5-11] that try to give an explanation of the liquidliquid transition phenomenon. In that case, the change of molecular ordering when passing through the liquidliquid transition is known to locally change the relative position of the molecular mass centers, and is associated with a transition from a low-density liquid to a high-density ordinary liquid. No change in the relative orientation of the molecules has been reported between the two liquid states. Such a liquid-liquid phase transition is one of the main puzzling hypothesis used to explain the properties of water [12].In a previous study, [13,14] we showed that liquid trans-dichloroethylene exhibits a continuous change in both dynamics and structure when studied as a function of temperature, i.e. there is a change form a high density to a low density liquid without the occurrence of a first order phase transition. The pri...

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Experimental and numerical signatures of dynamical crossover in orientationally disordered crystals

Cited by 21 publications

References 27 publications

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

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

Onset of slow dynamics in difluorotetrachloroethane glassy crystal

On the microscopic mechanism behind the purely orientational disorder–disorder transition in the plastic phase of 1-chloroadamantane

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