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

We measured the specific heat C_p 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 C_p ≈ T³. 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.