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

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