High frequency dynamics of an orientationally disordered molecular crystal

Angelini, Roberta; Scopigno, T.; Beraud, Alexandre; Ruocco, G.

doi:10.1016/j.jnoncrysol.2006.02.175

Cited by 6 publications

(4 citation statements)

References 15 publications

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“…[20][21][22][23][24][25] The pseudoglobular molecular shape of these compounds together with the dipolar character of the derivatives inferred by the substitution of one hydrogen in the adamantane molecule provides to this group interesting properties which can be used for fine tuning the required properties. Among the substances displaying an orientationally disordered ͑OD͒ phase, the adamantane derivatives form a large and interesting group.…”

Section: ␣-Relaxation Dynamics Of Orientanionally Disordered Mixed Crmentioning

confidence: 99%

α -relaxation dynamics of orientanionally disordered mixed crystals composed of Cl-adamantane and CN-adamantane

García

Tamarit

Capaccioli

et al. 2010

The Journal of Chemical Physics

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The alpha-relaxation dynamics of 1-cyano-adamantane (CNA) and its mixtures with 1-chloro-adamantane (ClA) has been studied by means of broadband dielectric spectroscopy. The existence of orientationally disordered (OD) face centered cubic mixed crystals (ClA(1-X)CNA(X)) for 0.5 < or = X < or = 1 has been put in evidence by thermodynamics and structural analyses. In addition to the OD phase of CNA, mixed crystals with compositions higher than the equimolar one exhibit a freezing of the orientational degrees of freedom into a glassy state, which involves also a strong increase of the antiferroelectric order at temperatures higher than the dielectric glass transition temperature. This experimental evidence is revealed by a stairlike effect in the variation of the Kirkwood factor with the temperature as a consequence of a twin effect in the dielectric strength without any anomaly in the temperature-density curves. The characteristic relaxation times are analyzed as a function of temperature and mole fraction. By setting a common temporal origin ("isochronal origin") at tau(T(g)) = 100 s for each mole fraction, it emerges that the substitution of ClA molecules by those of CNA (diminution of X) gives rise to a slow down in the dynamics, despite that the molecular volume of ClA molecules are smaller than those of CNA. This fact goes along and is accompanied by a diminution of the lattice packing with the decrease of composition. It is also shown that the heterogeneities produced by the concentration fluctuations due to the chemical disorder are the main contribution to the non-exponential character of the alpha-relaxation peaks.

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Section: ␣-Relaxation Dynamics Of Orientanionally Disordered Mixed Crmentioning

confidence: 99%

α -relaxation dynamics of orientanionally disordered mixed crystals composed of Cl-adamantane and CN-adamantane

García

Tamarit

Capaccioli

et al. 2010

The Journal of Chemical Physics

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“…CN-A has been studied by means of an extended number of experimental techniques [13,[17][18][19][20][21][22][23][24][25][26][27]. As far as its polymorphism is concerned, the low-temperature ordered phase is monoclinic (space group C2/m, Z = 4).…”

Section: Pure Compoundsmentioning

confidence: 99%

Dynamics of orientationally disordered mixed crystal sharing Cl-adamantane and CN-adamantane

García

Capaccioli

Díez

et al. 2010

Journal of Non-Crystalline Solids

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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].…”

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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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High frequency dynamics of an orientationally disordered molecular crystal

Cited by 6 publications

References 15 publications

α -relaxation dynamics of orientanionally disordered mixed crystals composed of Cl-adamantane and CN-adamantane

α -relaxation dynamics of orientanionally disordered mixed crystals composed of Cl-adamantane and CN-adamantane

Dynamics of orientationally disordered mixed crystal sharing Cl-adamantane and CN-adamantane

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

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