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

[Image: see text] 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 “Boson-peak” in the context of glassy crystals, is not exclusive of disordered materials.