| dc.description |
We use molecular dynamics computer simulations to study the relaxation dynamics of Na<sub>2</sub>O–2(SiO<sub>2</sub>) in its molten, highly viscous state. We find that at low temperatures the incoherent intermediate-scattering function for Na relaxes about 100 times faster than the one of the Si and O atoms. In contrast to this, all coherent functions relax on the same timescale if the wavevector is around 1 Å<sup>−1</sup>. This anomalous relaxation dynamics is traced back to the channel-like structure for the Na atoms that have been found for this system. We find that the relaxation dynamics for Si and O as well as the time dependence of the coherent functions for Na can be rationalized well by means of mode-coupling theory. In particular, we show that the diffusion constants as well as the α-relaxation times follow the power law predicted by the theory and that in the β-relaxation regime the correlators obey the factorization property with a master curve that is described well by a von Schweidler law. The value of the von Schweidler exponent b is compatible with the one found for the above-mentioned power law of the relaxation times/diffusion constants. Finally, we study the wavevector dependence of f<sub>s</sub>(q) and f(q), the coherent and incoherent non-ergodicity parameters. For the Si and O atoms these functions look qualitatively similar to the ones found for simple liquids and pure silica, in that the coherent function oscillates (in phase with the static structure factor) around the incoherent one and in that the latter is approximated well by a Gaussian function. In contrast to this, f(q) for Na–Na is always smaller than f<sub>s</sub>(q) for Na and the latter can be approximated by a Gaussian only for relatively large q. |
