Dynamics of glassy and liquid selenium

Abstract

Inelastic-neutron-scattering studies of glassy, liquid, and polycrystalline selenium have been performed at temperatures between 100 and 600 K. A self-consistent data evaluation, taking careful account of multiple and multiphonon scattering, shows that a complete interpretation is possible in terms of a temperature-dependent density of vibrational states, together with diffusion broadening of the elastic line. This density of states is used to calculate thermodynamic properties and to show that about one-third of the additional entropy of the liquid is vibrational. The results raise a number of questions concerning current theories of the glass transition.PACS numbers: 64.70.-p, 63.50.+X It has long been realized that a fundamental difference between a glass and a supercooled liquid is the presence in the latter of both conformational and vibrational motion. As an idealized example, motion in the liquid can be represented by an atom vibrating about an equilibrium position which is itself changing with time as a result of diffusion. What is not yet understood, however, is the way in which conformational motion freezes out at the glass transition temperature Tg and the way in which it influences thermodynamic properties. To take the particular example of interest here, there have been no direct measurements of the relative importance of conformational and vibrational contributions to the sudden (but not discontinuous) increase in the heat capacity CpiT) on passing from the glass to the supercooled liquid, * one of the most obvious signatures of Tg, That this increase in C{T) is not simply a "release of degrees of freedom" can be seen in Se^ where CpiT) increases from the classical value of 3/? mole ~ ^ K ~ ^ in the glass to a value 50% higher just above Tg. (Se is a glass with a Tg sufficiently low for easy study, but without bonded H atoms or side groups which complicate the analysis in organic polymers.)Recent mode-coupling theories ^' "^ have attempted to understand conformational motion (also known as relaxation) by describing it in terms of a time-dependent particle-density correlation function. The predictions of these theories have encouraged a number of neutron studies of the dynamics of the glass transition: ^"^ Conformational and vibrational motion are distinguished by time scales of typically 1 ns (varying rapidly with temperature but set by the experiment) and 1 ps, respectively, and show up in quasielastic and spin-echo experiments, on the one hand, and in measurements of the Debye-Waller factor and inelastic scattering, on the other. Most experiments have been concerned with conformational motion above Tg and demonstrate the presence of relaxation with a temperature dependence similar to that shown by the bulk viscosity, and in reasonable agreement with theory. Vibrational motion has been examined less carefully, mainly because detailed analysis of the data is complicated by multiple and multiphonon scattering. In this Letter we adopt a self-consistent, model-independent procedure for data analysis an...

Structure and relaxation in liquid and amorphous selenium

“…2͒, are in reasonable agreement with experiments on liquid Se. 44,45 At the higher temperatures D can be fitted by an Arrhenius law with an activation energy of 0.3 eV, in agreement with results from first-principle molecular dynamics simulations. 29 In the inset of Fig.…”

Vibrational dynamics and surface structure of amorphous selenium

“…The iteration terminates when numerical convergence of S 1 (Q, E) is achieved. Best agreement of the bulk VDoS measured by IXS and inelastic neutron-scattering experiments 19,20 is achieved for an elastic peak contribution of a 0 = 0.6, see Figure 3.…”

“…The VDoS were determined under the incoherent approximation using the structural factors at all available values of Q, assuming that single-phonon scattering is predominant. The comparison between the VDoS determined by IXS and inelastic neutron-scattering experiments 19,20 and molecular dynamics simulations 21 is shown in Figure 3 for bulk glassy Se. A good agreement has been achieved by selecting an elastic peak contribution of ~60%.…”

Correlations between Vibrational Entropy and Dynamics in Liquids

“…The latter effect implies that Eq. (1) is an approximation, the accuracy of which was tested using inelastic neutron scattering compared with calorimetric measurements in selenium [19], where it was shown to be accurate throughout the glass phase. This leads for the vibrational specific heat C vib :…”

“…The latter effect does not appear in high frequency dynamic heat capacities studies, as it requires changes of configurations that take place on the time scale τ to occur, but does contribute to the jump of specific heat characterizing the glass transition, whose amplitude is known to strongly correlate to the liquid fragility [16]. The configurational fraction of the jump in specific heat has been estimated using calorimetry in quenched and annealed glasses [15], measurements of elastic moduli [17], densities of states [18][19][20] and non-linear dielectric susceptibilities [21]. Overall, a great variation was found among glasses, with a configurational fraction ranging from 15% to 80% and apparently decaying with the liquid fragility [21].…”