The Boson peak in structural and orientational glasses of simple alcohols: specific heat at low temperatures

Abstract

We review in this work specific-heat experiments, that we have been conducted on different hydrogen-bonded glasses during recent years. Specifically, we have measured the low-temperature specific-heat C p for a set of glassy alcohols: normal and fully deuterated ethanol, 1- and 2-propanol, and glycerol. Ethanol exhibits a very interesting polymorphism presenting three different solid phases at low temperature: a fully ordered (monoclinic) crystal, an orientationally disordered (cubic) crystal or `orientational glass', and the ordinary structural glass. By measuring and comparing the low-temperature specific heat of the three phases, in the `Boson peak' range 2–10 K as well as in the tunneling-states range below 1 K, we are able to provide a quantitative confirmation that `glassy behavior' is not an exclusive property of amorphous solids. On the other hand, propanol is the simplest monoalcohol with two different stereoisomers (1- and 2-propanol); this allows us to study directly the influence of the spatial rearrangement of atoms on the universal properties of glasses. We have measured the specific heat of both isomers, finding a noteworthy quantitative difference between them. Finally, low-temperature specific-heat data of glassy glycerol have also been obtained. Here we propose a simple method based upon the soft-potential model to analyze low-temperature specific-heat measurements, and we use this method for a quantitative comparison of all these data from glassy alcohols and as a stringent test of several universal correlations and scaling laws suggested in the literature. In particular, we find that the interstitialcy model for the Boson peak [Phys. Rev. Lett. 68 (1992) 974] gives a very good account of the temperature T max at which the maximum in C p/ T 3 occurs.