Evangelia A. Pavlatou

E.A. Pavlatou, G.N. Papatheodorou, A.K. Rizos, G. Fytas

The density correlation function, C(t), in the mixed ionic glass KNO₃·Ca(NO₃)₂ with three different compositions between 50 and 70% at temperatures near and above the glass transition, T_g, was measured by photon correlation spectroscopy. The C(t) is well represented by non-exponential form exp(−t/τ)^β with β=0.47±0.03 insensitive to temperature and composition variation in the investigated range. The relaxation time, τ, is independent of the wavevector, q, in the range (1.2–4.7) × 10⁻³Å⁻¹ and its dependence on the composition arises mainly from the composition-dependent T_g. The relaxation time which is in agreement with recent neutron spin-echo (NSE) (time domain 2×10⁻¹¹−2×10⁻⁹s) is associated with local rearrangements involving a few neighbouring ions. Information about local structure can be extracted from the low-frequency Raman spectra.

doi: 10.1016/0022-3093(91)90279-F

E.A. Pavlatou, G.N. Papatheodorou, A.K. Rizos, G. Fytas

We report spontaneous Brillouin and photon correlation spectroscopic results on the dynamics of the local density fluctuations in the ionic mixed salt xKNO₃⋅yCa(NO₃)₂ with molar ratios 2:1, 3:2, and 1:1. For these mixtures, the density relaxation function g(t) changes from a stretched exponential (β=0.47) near the glass transition temperature Tg, to an exponential (β=1) form at high temperatures, whereas the composition dependence of the relaxation time τ reflects mainly differences between the T_g values. The 3:2 fragile glass (T_g=60°C) is one of the few examples where g(t) is now known from the glass‐like (60–90 °C) to the liquid‐like (T>100 °C) region through the application of various dynamic scattering techniques. The τ(T) and the evolution of the width β of the distribution are discussed in terms of the physical pictures implied by current models of the liquid and glass transition phase.

doi: 10.1063/1.460731