Low-temperature specific heat and glassy dynamics of a polymorphic molecular solid

Abstract

The low-frequency dynamics of different solid phases of ethanol ͑fully ordered crystal, orientational glass, and true structural glass͒ is investigated by measurements of low-temperature specific heat as well as determination of the vibrational densities of states by inelastic neutron scattering. Such polymorphism provides a unique benchmark to study the relative importance of different kinds of disorder on glass-dynamics properties. The measurements are carried out both for hydrogenous and fully-deuterated samples. Large isotopic effects are found for the excess contributions ͑i.e., non-Debye͒ to the specific heat of the disordered solids, which have clear correlates in the low-frequency spectra. ͓S0163-1829͑98͒05626-4͔

Scattering of acoustic phonons in disordered matter: A quantitative evaluation of the effects of positional versus orientational disorder

“…This enables us to track down the dispersion relation for acoustic phonons throughout reciprocal space, an issue that cannot be pursued on polycrystalline samples. 13 As a point of departure we will here consider the lattice dynamics model for the FOC, 14 which was scrutinized against the experimentally derived vibrational densities of states and found to provide an adequate representation of the harmonic crystal dynamics up to about 25 meV. Calculated phonon dispersion branches along the highest-crystalsymmetry directions show that acoustic phonons are bounded below Ϸ8 meV.…”

“…2. The lowest temperature ͑5 K͒ corresponds to the upper limit of the strong rise with temperature of ͑T͒; T = 30 K is the limiting temperature where the Debye model is able to account for the measured specific heat, 14 or put into different words, where the specific heat can be fully accounted for in terms of acoustic excitations only. Finally, T = 100 K comes close to the stability limit of the ODC and also to the actual temperature where IXS measurements were conducted.…”

“…Here we show that thermal disorder significantly shortens the lifetimes of acoustic excitations within the ODC, which leads to significantly smaller heat conductivities, in agreement with experiment. To reconcile both results it is worth recalling that IXS ͑or INS͒ experiments carried out on polycrystals 5 mostly sample inhomogeneous spectra composed of excitations additional to those of acoustic character, 14 which preempts the interpretation of maxima and linewidths of the spectral bands in terms of physical frequencies and their damping terms.…”

“…4,15 Such frequencies are, however, confined to values not far in excess of that of the BP at high temperatures. As far as identification of the prime entities responsible for scattering of acoustic phonons, low-energy torsional librations about the C-C bond 14 are deemed to be the main contributors to such processes. As shown previously for other materials 16 scattering from such excitations leads to a plateau in ͑T͒ and also reproduces the observed trends up to relatively high temperatures.…”

Ethanol Glass Dynamics: Logarithmic Line Broadening and Optically Induced Dephasing

“…Both ethanol and deuterated ethanol can also be prepared as a stable orientational glass. [61][62][63][64] In this phase the molecules are arranged on an ordered lattice but with disordered orientations. Even though the structural glass was used for the experiments here, positional disorder is not essential for the manifestation of glasslike behavior.…”