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

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.