Low-Temperature Heat Capacity Anomalies in Ordered and Disordered Phases of Normal and Deuterated Thiophene

We measured the specific heat Cp of normal (C4H4S) and deuterated (C4D4S) thiophene in the temperature interval of 1 = T, K = 25. C4H4S exhibits a metastable phase II2 and a stable phase V, both with frozen orientational disorder (OD), whereas C4D4S exhibits a metastable phase II2, which is analogous to the OD phase II2 of C4H4S and a fully ordered stable phase V. Our measurements demonstrate the existence of a large bump in the heat capacity of both stable and metastable C4D4S and C4H4S phases at temperatures of ~10 K, which significantly departs from the expected Debye temperature behavior of Cp ˜ T3. This case study demonstrates that the identified low-temperature Cp anomaly, typically referred to as a “Boson-peak” in the context of glassy crystals, is not exclusive of disordered materials.Peer ReviewedPostprint (published version

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

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.Peer ReviewedPostprint (published version

Glassy Anomalies in the Low-Temperature Thermal Properties of a Minimally Disordered Crystalline Solid

The low-temperature thermal and transport properties of an unusual kind of crystal exhibiting minimal molecular positional and tilting disorder have been measured. The material, namely, low-dimensional, highly anisotropic pentachloronitrobenzene has a layered structure of rhombohedral parallel planes in which the molecules execute large-amplitude in-plane as well as concurrent out-of-plane librational motions. Our study reveals that low-temperature glassy anomalies can be found in a system with minimal disorder due to the freezing of (mostly in-plane) reorientational jumps of molecules between equivalent crystallographic positions with partial site occupation. Our findings will pave the way to a deeper understanding of the origin of the above-mentioned universal glassy properties at low temperature.Peer ReviewedPostprint (published version

Thermal Conductivity of Triphenyl Phosphite's Liquid, Glassy, and Glacial States

The thermal conductivity kappa and heat capacity per unit volume rho C-p of triphenyl phosphite (TPP) were measured under different pressure and temperature conditions, and with time during the sluggish liquid to glacial state transformation at temperatures about 15 K above the glass transition temperature. As the transformation slowly proceeds during several hours, rho C-p decreases monotonically from that of the liquid state to a value close to that of the vitrified state. Concurrently, kappa increases nonmonotonically with an intermediate local maximum followed by a minimum, before the final rise to a higher kappa. The properties of the ultimately formed glacial state depend on the thermal history, which implies that the state formed under these conditions is a heterogeneous mixture of nanocrystals and mainly amorphous-like solid, and that the relative amount and microstructure depend on the conditions of the transformation. The nonmonotonic changes in phonon propagation during the liquid to glacial transformation suggest microstructural changes which are consistent with a liquid-liquid transformation and sluggish growth of nanocrystals within amorphous-like solid domains. The isobaric thermal conductivity of the as-formed glacial state shows a reversible step increase, just prior to crystallization on heating, which deviates from the typical behavior of glasses, liquids, and crystals. An increase in pressure shifts the step to higher temperatures and suppresses crystallization, which reveals another reversible rise in kappa and C-p. These results show that increased molecular mobility in the glacial state increases and suggest reduced thermal resistance at boundaries or that the motions carry heat

Polymorphic states investigations in thermal conductivity of 1-fluoroadamantane

The temperature dependencies of the thermal conductivity coefficient, κ (T), in 1-fluoroadamantane were investigated for two different orientationally-disordered states. In the first one, κ (T) exhibits a crystalline character with a typical maximum and two anomalous regions marked at higher temperatures. Here, the low-temperature power-law dependence is reduced due to a relatively large contribution of dislocations and defects in the polycrystalline sample. Above 196 K, there is an anomalous plateau in thermal conductivity resulting from the presence of an intermediate centrosymmetric state. Further, a significant increase in κ occurs above 234 K, resulting from a phase transition to a high-temperature disordered cubic phase. Subsequent cooling of the sample leads to an irreversible transition towards a disordered state resulting in a κ (T) dependence corresponding to an amorphous material behavior. The primary anomaly at 196 K disappears, while the second one associated with the phase transition at 227 K shifts towards lower temperatures. The irreversibility of this transformation is also verified by scanning microscopy images. Thermal conductivity in both phases has an additional contribution of an Arrhenius type in their temperature dependences

Thermal conductivity of solid triphenyl phosphite

The thermal conductivity, κ, of solid triphenyl phosphite was measured by using the transient hot-wire method, and its temperature and pressure dependencies were analyzed to understand heat transfer processes in the solid polymorphic phases, as well as in the glass and the exotic glacial state. Phase transformations and the structural order of the phases are discussed, and a transitional pressure–temperature diagram of triphenyl phosphite is presented. The thermal conductivity of both the crystalline and disordered states is described within the theory of two-channel heat transfer by phonons and diffusons in dielectric solids. In the glass and glacial states, the weakly temperature-dependent (glass-like) κ is described well by the term associated with heat conduction of diffusons only, and it can be represented by an Arrhenius-type function. In the crystal phases, the strongly temperature-dependent (crystal-like) κ associated with heat transfer by phonons is weakened by significant heat transfer by diffusons, and the extent of the two contributions is reflected in the temperature dependence of κ. We find that the contribution of diffusons in the crystal phases depends on pressure in the same way as that in amorphous states, thus indicating that the same mechanism is responsible for this channel of heat transfer in crystals and amorphous states

Influence of thermal treatment on thermal properties of adamantane derivatives

Heat transport mechanisms present in 2-adamantanone and 1-cyanoadamantane crystals were investigated in a broad temperature range. To characterize scattering processes, thermal conductivity and heat capacity measurements were carried out. A particular care was paid to the cooling rate of specimen which influenced the thermal history of the samples. The experimental results led to a conclusion that under slow cooling the thermal conductivity reaches the highest values and resembles the behavior of ordered molecular crystals. As for fast cooling, thePostprint (author's final draft

Anomalous behavior of thermal conductivity at high temperatures for molecular crystals composed of flexible molecules

The temperature dependence of thermal conductivity ¿(¿) of selected molecular polycrystals consisting of flexible molecules was investigated within 20–260¿K. The materials were para-chloronitrobenzene, pentachloronitrobenzene and freons F-112 and F-113. The ¿(¿) of these crystals increased with temperature in the interval where processes of phonon–phonon scattering were dominant. The increase was observed both in the orientationally-ordered and -disordered phases and is typical of the thermally activated heat transfer by localized molecular excitations [M. A. Strzhemechny et al., Chem. Phys. Lett. 647 (2016) 55]. In a wide interval of temperatures, irrespective of the glass transition temperature, the ¿(¿) could be described by a sum of three components: ¿(¿)¿=¿¿/¿ + ¿ + ¿¿¿(¿). The term A/T accounts for phonon–phonon scattering processes and B accounts for diffuse phonon scattering. The third contribution is described by the Arrhenius equation, ¿¿¿(¿)¿=¿¿0 ¿¿¿(-¿/kB¿), where E is the activation energy and ¿0 is the pre-exponential factor characterizing intensity of the activation process. A comparative analysis of anomalous thermal conductivities of some other molecular crystals was carried out. It was found that ¿0 linearly depended on E and a similar relationship was evident for a series of quasicrystals.Peer ReviewedPostprint (author's final draft