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

Heat capacity C-p(T) 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 (theta(D) = 560 K, theta(E1) = 130 K, and theta(E2) = 95 K) of the thermal characteristics of Ge31P15Se8 are related to concerted and localized vibrations of different fragments of the semiclathrate crystal structure. (C) 2016 Elsevier B. V. All rights reserved.