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

Abstract

We measured the specific heat C p of normal (C 4 H 4 S) and deuterated (C 4 D 4 S) thiophene in the temperature interval of 1 ≤ T, K ≤ 25. C 4 H 4 S exhibits a metastable phase II 2 and a stable phase V, both with frozen orientational disorder (OD), whereas C 4 D 4 S exhibits a metastable phase II 2 , which is analogous to the OD phase II 2 of C 4 H 4 S 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 C 4 D 4 S and C 4 H 4 S phases at temperatures of ∼10 K, which significantly departs from the expected Debye temperature behavior of C p ≈ T 3 . This case study demonstrates that the identified low-temperature C p anomaly, typically referred to as a "Bosonpeak" in the context of glassy crystals, is not exclusive of disordered materials.

Role of Optical Phonons and Anharmonicity in the Appearance of the Heat Capacity Boson Peak-like Anomaly in Fully Ordered Molecular Crystals

“…■ 76 red stars; deuterated ethanol 77 for the orientational glass OG (dark green points), structural glass SG (light green line), and fully ordered crystal (light green points); glycerol, 78 structural glass (pink line) and ordered crystal (pink points); deuterated (fully ordered, half-filled black circles) and normal (disordered, half-filled red circles) low-temperature phases of thiophene 16 (filled green stars); CCl 4 , ordered monoclinic phase. 14 The Journal of Physical Chemistry Letters…”

“…However, during the past decade several experimental and molecular dynamics studies have also evidenced the existence of glass-like C p anomalies in perfectly ordered and minimally disordered molecular crystals. − These findings suggest that the physical causes of the described anomalies should be more general than previously thought and not exclusive of glasses. It is therefore reasonable to think that by improving our physical understanding of molecular crystals exhibiting minimal or null disorder, which can be carefully and thoroughly analyzed with well-established experimental and computational techniques, we can clarify the apparently “universal” character of the BP and progress in our unsatisfactory comprehension of glasses.…”

“… Experimental heat capacity data for benzophenone and its bromine-substituted derivatives (symbols as in Figure 1 ) expressed in the normalized coordinates [( C p – C D )/ T 3 ]/[( C p – C D )/ T 3 ] max and as a function of T / T max . Literature data: Ar, 76 red stars; deuterated ethanol 77 for the orientational glass OG (dark green points), structural glass SG (light green line), and fully ordered crystal (light green points); glycerol, 78 structural glass (pink line) and ordered crystal (pink points); deuterated (fully ordered, half-filled black circles) and normal (disordered, half-filled red circles) low-temperature phases of thiophene 16 (filled green stars); CCl 4 , ordered monoclinic phase. 14 …”

Low-Frequency Vibrational Density of State of Janus-Polynorbornenes: The Dependence of the Boson Peak on the Nanophase-Separated Structure

“…Generally, there is a consensus in the literature that the BP is a universal feature of amorphous materials. However, it is worth noting that a BP is also observed for materials, which have a partial order, e.g., plastic crystals (see for instance refs − ). Although the underlying time scales are different (about 10 2 s at the thermal glass transition temperature T g and in the range of picoseconds for the BP), the BP might have some implications for the glass transition. , There is some agreement in the literature that the glass transition is due to cooperative effects. − This approach was pioneered by Adam and Gibbs introducing the concept of cooperatively rearranging regions (CRR) .…”

Dynamics and local ordering of pentachloronitrobenzene: a molecular-dynamics investigation

“…The molecules rather perform reorientational jumps between well-defined distinguishable orientations compatible with the symmetry operations of the site of the lattice in which they are located. 2–8…”