Secondary relaxation in the terahertz range in 2-adamantanone from theory and experiments

Abstract

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]

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

“…The disordered character of the LT phase induces the appearance of glassy C p features commonly found in structural (canonical) and orientational glasses, like the boson peak in the C p / T 3 versus T representation. Such anomalous C p features originated by F occupational disorder are very similar to those previously found in systems presenting occupational and orientational disorder (i.e., 2-adamantanone 25 , 45 and cyano-adamantane, respectively). Moreover, regardless the type of disorder, the maximum temperature of the boson peak appears to be directly correlated with the energy of the first optical mode (“soft-mode”) appearing at the center of the Brillouin zone of the crystal.…”

“…Although the corresponding phonon dispersion curves are not available, it can be hypothesized based on their molecular correspondence that the energy of the lowest-energy optical modes is quite similar for all them (even though the translational-rotational coupling of the low-energy localized optical modes with acoustic propagating phonons may differ). Such a hypothesis is actually valid for 2-adamantanone, as it has been experimentally demonstrated for its monoclinic phase through analysis of its vibrational eigenvectors 45 . Within the same line of reasoning, the similarity between the vibrational density of states (VDOS), g (ω) , of some of the aforementioned compounds is also evidenced when the g (ω)/ω 2 function is normalized to its maximum value and represented as a function of ω/ω max , where ω max is the frequency at which the g (ω)/ω 2 maximum appears (see Fig.…”

Johari–Goldstein Heterogeneous Dynamics in a Model Polymer

“…Finally, by inspecting both ISF and MSD in Figure , one sees that the chains with shorter bonds exhibit smaller monomer MSD in regions I and III and slower structural relaxation in region III, as previously reported . The role of ISF to model the memory kernel in a generalized Langevin equation theory dealing with both primary and JG relaxations has been emphasized. − …”

Johari-Goldstein heterogeneous dynamics in a model polymer

“…47 The role of ISF to model the memory kernel in a generalized Langevin equation theory dealing with both primary and JG relaxations has been emphasized. [51][52][53][54] The finding that, in the presence of JG relaxation, DH built up in the JG regime persists up to the α relaxation, see Fig. 4, conveys the impression of the possible interplay of DHs in the JG and the long-time regimes, also on the basis that long-time DH is sensed at short times, 55 including JG time scale for non-polymeric systems, 26 and even at vibrational time scales.…”