Both structural glasses and disordered crystals are known to exhibit anomalous thermal, vibrational and acoustic properties at low temperatures or low energies, what is still a matter of lively debate. To shed light on this issue, we have studied the halomethane family CBrnCl4−n (n = 0, 1, 2) at low temperature where, despite being perfectly translationally-ordered stable monoclinic crystals, glassy dynamical features had been reported from experiments and molecular dynamics simulations. For n = 1, 2 dynamic disorder originates by the random occupancy of the same lattice sites by either Cl or Br atoms, but not for the ideal reference case of CCl4. Measurements of the low-temperature specific heat (Cp) for all these materials are here reported, which provide evidence of the presence of a broad peak in Debye-reduced Cp(T )/T 3 and in the reduced density of states (g(ω)/ω 2 ) determined by means of neutron spectroscopy, as well as a linear term in Cp usually ascribed in glasses to two-level systems in addition to the cubic term expected for a fully-ordered crystal. Being CCl4 a fully-ordered crystal, we have also performed density functional theory (DFT) calculations, which provide unprecedented detailed information about the microscopic nature of vibrations responsible for that broad peak, much alike the "boson peak" of glasses, finding it to essentially arise from a piling up (at around 3 − 4 meV) of low-energy optical modes together with acoustic modes near the Brillouin-zone limits.

The specific heat of toluene in glass and crystal states, has been measured both at low temperatures down to 1.8 K (using the thermal relaxation method) and in a wide temperature range up to the liquid state (using a quasiadiabatic continuous method). Our measurements therefore extend earlier published data to much lower temperatures, thereby allowing to explore the low-temperature "glassy anomalies" in the case of toluene. Surprisingly, no indication of the existence of tunneling states is found, at least within the temperature range studied. At moderate temperatures, our data either for the glass or for the crystal show good agreement with those found in the literature. Also, we have been able to prepare bulk samples of toluene glass by only doping with 2% mol ethanol instead of with higher impurity doses used by other authors.

The specific heat C p of toluene, doped with 2 mol% ethanol to avoid rapid crystallization, has been measured in both glass and crystal states, and with special accuracy at low temperatures in the range 1.8−20 K using the thermal relaxation method. By making use of the complementary C p curves measured in the reference crystal state, we have been able to obtain the entropy curve of the glass and eventually the residual entropy of toluene glass in the zero-temperature limit, that is found to be 5.1 J/(K⋅mol). This value is clearly lower than others previously reported in the literature, which lack the knowledge of the particular specific-heat behavior of glasses at low temperatures and hence overestimated the glass residual entropy at zero temperature. In addition, we have studied in detail such low-temperature "glassy anomalies" in the case of toluene, extending and improving previous measurements. The surprising depletion previously reported of tunneling two-level systems in toluene glass has been confirmed, though this fact coexists with the presence of a broad peak typical of glasses (the socalled boson peak) in C p /T 3 at 4.5 K. For the toluene crystal, the expected cubic Debye behavior has been found at lower temperatures.

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Recent findings of structural glasses with extremely high kinetic and thermodynamic stability have attracted much attention. The question has been raised as to whether the well-known, low-temperature “glassy anomalies” (attributed to the presence of two-level systems [TLS] and the “boson peak”) persist or not in these ultrastable glasses of much lower configurational entropy. To resolve previous contradictory results, a particular type of ultrastable glass has been studied, TPD, which can be prepared by physical vapor deposition in a highly-stable state with different degrees of layering and molecular orientation, and also as a conventional glass and in crystalline state. After a thorough characterization of the different samples prepared, their specific heat was measured down to 0.4 K. Whereas the conventional glass exhibited the typical glassy behavior and the crystal the expected Debye cubic dependence at very low temperatures, a strong depletion of the TLS contribution was found in both kinds of ultrastable glass, regardless of their layering and molecular ordering.