Negative thermal expansion and low-temperature heat capacity anomalies of Ge31P15Se8 semiclathrate

• Heat capacity and thermal expansion of the Ge31P15Se8 were studied at 2–300 K.
• A negative expansion caused by guest atoms transitions between non-equivalent states.
• The characteristic temperatures are related to crystal lattice fragments.

Heat capacity Ср(Т) and thermal expansion V(T) of the Ge31P15Se8 semiclathrate were studied in the temperature range of 2–300 K. It is shown that the total heat capacity takes remarkably high values at low temperatures, exceeding 1 J mol−1 K−1, owing to a glass-like contribution arisen from irregularities in the structure of the clathrate framework. A Schottky-like term is subtracted and attributed to asymmetry of the guest atom environment inside the distorted polyhedral cages of the framework. At temperatures below 30 K, a negative thermal expansion was found possibly caused by guest atomic transitions between energetically non-equivalent states within distorted cages of the framework. Values of the Debye and Einstein terms (θD = 560 K, θE1 = 130 K, and θE2 = 95 K) of the thermal characteristics of Ge31P15Se8 are related to concerted and localized vibrations of different fragments of the semiclathrate crystal structure.