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

Abstract

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...

Simultaneous Orientational and Conformational Molecular Dynamics in Solid 1,1,2-Trichloroethane

“…The short lifetime of the intermediate state reached during a fluctuation rationalizes the fact that no significant distortion is detected by XRD analysis . In this sense, dielectric spectroscopy and molecular dynamics simulations are much more powerful than XRD (which measures only time-average structures) in detecting molecular dynamics of apparently ordered solids. − …”

“…14 In this sense, dielectric spectroscopy and molecular dynamics simulations are much more powerful than XRD (which measures only timeaverage structures) in detecting molecular dynamics of apparently ordered solids. [46][47][48][49][50][51][52] Since the relaxation processes are related to variations of the dipole moments of the individual Four time intervals can be identified in panel (a), labeled from i to iv. It can be seen that, in each time interval, the chlorines occupy most of the time equilibrium positions, corresponding to a stable molecular configuration, and that in between these comparatively long intervals, a configurational leap takes place (corresponding to relaxation I).…”

Heat capacity anomalies of the molecular crystal 1-fluoro-adamantane at low temperatures

“…This is the typical case of order–disorder phase transitions 2 , 3 , in which the involved phases show no apparent volume and entropy differences at the transition points, in contrast to first-order phase transitions. A particular class of continuous phase transition that is quite rare are solid–solid disorder-disorder phase transitions 4 .…”

“…All 1-X- and 2-X-adamantane species display rich polymorphism and a large number of experimental and theoretical investigations have been devoted to their study 5 – 9 . A common characteristic of 1-X- and 2-X-adamantane derivatives is that they form orientationally disordered (OD) crystals, in which translationally ordered molecules perform reorientational jumps over well-defined orientations 4 , 10 – 12 .

Figure 1 Molar heat capacity ( C p ) of 1-fluoro-adamantane expressed as a function of temperature.

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Diffusion of sodium ions in amorphousNa2Si2O5: Quasielastic neutron scattering andab initiomolecular dynamics s

“…However, from Fig. 3, the change in elastic intensity is more than that expected from only the Debye-Waller factor, which could be due to increase in disorder 74 .…”