Abstract:The transition taking place between two metastable phases in 2-O-adamantane, namely the [Formula: see text] cubic, rotator phase and the lower temperature P2/c, Z = 4 substitutionally disordered crystal is studied by means of muon spin rotation and relaxation techniques. Measurements carried out under zero, weak transverse and longitudinal fields reveal a temperature dependence of the relaxation parameters strikingly similar to those exhibited by structural glass[Formula: see text]liquid transitions (Bermejo… Show more
“…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. − …”
The molecular dynamics in the ambient-pressure solid phase of (1,1,2)trichloroethane is studied by means of broadband dielectric spectroscopy and molecular dynamics simulations. The dielectric spectra of polycrystalline samples obtained by crystallization from the liquid phase exhibit, besides a space-charge relaxation associated with accumulation of charges at crystalline domain boundaries, two loss features arising from dipolar 2 molecular relaxations. The most prominent and slower of the two loss features is identified as a configurational leap of the molecules which involves a simultaneous change in spatial orientation and structural conformation, namely between two isomeric forms (gauche + and gauche -) of opposite chirality. In this peculiar dynamic process, the positions of the three chlorine atoms in the crystal lattice remain unchanged, while those of the carbon and hydrogen atoms are modified.This dynamic process is responsible for the disorder observed in an earlier x-ray diffraction study and confirmed by our simulation, which is present only at temperatures relatively close to the melting point, starting 40 K below. The onset of the disorder is visible as an anomaly in the temperature dependence of the dc conductivity of the sample at exactly the same temperature.While the slower relaxation dynamics (combined isomerization/reorientation) becomes increasingly more intense on approaching the melting point, the faster dynamics exhibits significantly lower but constant dielectric strength. Based on our molecular dynamics simulations, we assign the faster relaxation to large fluctuations of the molecular dipole moments, partly due to large-angle librations of the chloroethane species.
“…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. − …”
The molecular dynamics in the ambient-pressure solid phase of (1,1,2)trichloroethane is studied by means of broadband dielectric spectroscopy and molecular dynamics simulations. The dielectric spectra of polycrystalline samples obtained by crystallization from the liquid phase exhibit, besides a space-charge relaxation associated with accumulation of charges at crystalline domain boundaries, two loss features arising from dipolar 2 molecular relaxations. The most prominent and slower of the two loss features is identified as a configurational leap of the molecules which involves a simultaneous change in spatial orientation and structural conformation, namely between two isomeric forms (gauche + and gauche -) of opposite chirality. In this peculiar dynamic process, the positions of the three chlorine atoms in the crystal lattice remain unchanged, while those of the carbon and hydrogen atoms are modified.This dynamic process is responsible for the disorder observed in an earlier x-ray diffraction study and confirmed by our simulation, which is present only at temperatures relatively close to the melting point, starting 40 K below. The onset of the disorder is visible as an anomaly in the temperature dependence of the dc conductivity of the sample at exactly the same temperature.While the slower relaxation dynamics (combined isomerization/reorientation) becomes increasingly more intense on approaching the melting point, the faster dynamics exhibits significantly lower but constant dielectric strength. Based on our molecular dynamics simulations, we assign the faster relaxation to large fluctuations of the molecular dipole moments, partly due to large-angle librations of the chloroethane species.
“…The polymorphic behavior of this compound has been described many times in the literature in the past 22,[30][31][32] . The OD room temperature phase melts at 529 K and exhibits a face-centered-cubic (FCC) structure (space group F m 3m.)…”
We applied the recently developed Generalized Langevin Equation (GLE) approach for dielectric response of liquids and glasses to link the vibrational density of states (VDOS) to the dielectric response of a model orientational glass (OG). The dielectric functions calculated based on the GLE, with VDOS obtained in experiments and simulations as inputs, are compared with experimental data for the paradigmatic case of 2-adamantanone at various temperatures. The memory function is related to the integral of the VDOS times a spectral coupling function γ(ωp), which tells the degree of dynamical coupling between molecular degrees of freedom at different eigenfrequencies. With respect to previous empirical fittings, the GLE-based fitting reveals a broader temperature range over which the secondary relaxation is active. Furthermore, the theoretical analysis provides a clear evidence of secondary relaxation being localized within the THz (0.5 − 1 THz) range of eigenfrequencies, and thus not too far from the low-energy modes involved in α-relaxation. In the same THz region, the same material displays a crowding of low-energy optical modes that may be related to the secondary relaxation. arXiv:2003.09627v1 [cond-mat.soft]
“…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.…”
Disorder–disorder phase transitions are rare in nature. Here, we present a comprehensive low-temperature experimental and theoretical study of the heat capacity and vibrational density of states of 1-fluoro-adamantane (C10H15F), an intriguing molecular crystal that presents a continuous disorder–disorder phase transition at T = 180 K and a low-temperature tetragonal phase that exhibits fractional fluorine occupancy. It is shown that fluorine occupancy disorder in the low-T phase of 1-fluoro-adamantane gives rise to the appearance of low-temperature glassy features in the corresponding specific heat (i.e., “boson peak” -BP-) and vibrational density of states. We identify the inflation of low-energy optical modes as the main responsible for the appearance of such glassy heat-capacity features and propose a straightforward correlation between the first localized optical mode and maximum BP temperature for disordered molecular crystals (either occupational or orientational). Thus, the present study provides new physical insights into the possible origins of the BP appearing in disordered materials and expands the set of molecular crystals in which “glassy-like” heat-capacity features have been observed.
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