Thermal properties of halogen-ethane glassy crystals: Effects of orientational disorder and the role of internal molecular degrees of freedom

Abstract

10 págs.; 6 figs.; 4 tabs. ; © 2015 AIP Publishing LLC. The thermal conductivity, specific heat, and specific volume of the orientational glass former 1,1,2-trichloro-1,2,2-trifluoroethane (CCl2F–CClF2, F-113) have been measured under equilibrium pressure within the low-temperature range, showing thermodynamic anomalies at ca. 120, 72, and 20 K. The results are discussed together with those pertaining to the structurally related 1,1,2,2-tetrachloro-1,2-difluoroethane (CCl2F–CCl2F, F-112), which also shows anomalies at 130, 90, and 60 K. The rich phase behavior of these compounds can be accounted for by the interplay between several of their degrees of freedom. The arrest of the degrees of freedom corresponding to the internal molecular rotation, responsible for the existence of two energetically distinct isomers, and the overall molecular orientation, source of the characteristic orientational disorder of plastic phases, can explain the anomalies at higher and intermediate temperatures, respectively. The soft-potential model has been used as the framework to describe the thermal properties at low temperatures. We show that the low-temperature anomaly of the compounds corresponds to a secondary relaxation, which can be associated with the appearance of Umklapp processes, i.e., anharmonic phonon-phonon scattering, that dominate thermal transport in that temperature range. ; This work was financially supported in part by the Spanish Ministry of Science and Innovation (Grant Nos. FIS2014-54734-P, FIS2011-23488, and MAT2014-57866- REDT), by the Catalan Government (Grant No. 2014SGR- 0581) and by the Comunidad de Madrid through program NANOFRONTMAG-CM (No. S2013/MIT-2850), as well as by the joint NAS Ukraine and Russian Foundation for Basic Research project "Metastable states of simple condensed systems" (Agreement No. N 7/-2013). ; Peer Reviewed