Frequency-dependent specific heat of viscous silica

Abstract

We apply the Mori-Zwanzig projection operator formalism to obtain an expression for the frequency dependent specific heat c(z) of a liquid. By using an exact transformation formula due to Lebowitz et al., we derive a relation between c(z) and K(t), the autocorrelation function of temperature fluctuations in the microcanonical ensemble. This connection thus allows to determine c(z) from computer simulations in equilibrium, i.e. without an external perturbation. By considering the generalization of K(t) to finite wave-vectors, we derive an expression to determine the thermal conductivity λ from such simulations. We present the results of extensive computer simulations in which we use the derived relations to determine c(z) over eight decades in frequency, as well as λ. The system investigated is a simple but realistic model for amorphous silica. We find that at high frequencies the real part of c(z) has the value of an ideal gas. c ′ (ω) increases quickly at those frequencies which correspond to the vibrational excitations of the system. At low temperatures c ′ (ω) shows a second step. The frequency at which this step is observed is comparable to the one at which the α−relaxation peak is observed in the intermediate scattering function. Also the temperature dependence of the location of this second step is the same as the one of the α−peak, thus showing that these quantities are intimately connected to each other. From c ′ (ω) we estimate the temperature dependence of the vibrational and configurational part of the specific heat. We find that the static value of c(z) as well as λ are in good agreement with experimental data.

Structural properties of molten silicates fromab initiomolecular-dynamics simulations: Comparison betweenCaO−Al2

“…This value does not disagree with an extrapolation of the experimental values of C p from Ref. [56] and is close to the values obtained by Scheidler et al [57] from molecular dynamics simulation of the SiO 2 melt in the same temperature range, using the BKS potential [35]. Scheidler et al computed the frequency dependent specific heat from T =2750 K up to T =6100 K and found a reasonable extrapolation of the experimental C p (T ) above T g .…”

Liquid–liquid transition in a strong bulk metallic glass-forming liquid

“…3 of Scheidler et al 52 ). The heat capacity values are plotted against the T g -scaled temperature.…”

Amorphous silica modeled with truncated and screened Coulomb interactions: A molecular dynamics simulation study

“…Thus, we shall first fix the value of the cutoff r c , and then determining the best set of parameters for D Y and ∆ to minimize the function in Eq. (8). This procedure can then be repeated for different values of the cutoff r c , and for different temperatures.…”

“…It is the basic oxidic component of many minerals and technological glasses. In recent years, molecular dynamics (MD) computer simulations have provided valuable insight into static and dynamic properties of amorphous and crystalline silica [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25]. In a classical MD simulation, the interactions between the atoms are described by an effective potential where, different from ab initio approaches, the electronic degrees of freedom are not taken into account explicitly.…”