S.N. Yannopoulos, E.A. Pavlatou

Chimica Chronica-New Series (1994), 23, 257-263

Photon correlation and Brillouin spectroscopy have been applied in the glass forming systems ZnCl2, ZnBr2 and the 50% ZnCl2-ZnBrmixture, in order to study the dynamics of local density fluctuations below Tg and far above Tm. The density correlation function of pure ZnCl2 near Tg is well represend by the non-exponential form exp(-t/τ)β with β=0.706±0.012 insensitive to temperature. The high value of β parameter categorizes ZnCl2 in the class of relative "strong" glasses with an Arrhenius temperature dependence of relaxation time τ near Tg (Tg+80°C). The hypersonic properties of pure components and the 50% mixture were also studied in the GHz region, where a single relaxation process was revealed (β=1). The limiting sound velocities uo and u∞ were calculated, showing up a linear decrease with temperature. The longitudinal relaxation time τi was found to follow an Arrhenius type relation.

S.G. Kalogrianitis, T.G. Mihopoulos, E.A. Pavlatou, G.N. Papatheodorou

Reduced isotropic and anisotropic Raman spectra were measured for a series of molten alkali metal halides and their mixtures. The systems involving CsX and LiX (X=Cl, Br, I) were investigated at different temperatures and compositions. The spectra of the common anion mixtures show two bands which appear to be a superposition of the bands of the component salts. Similar is the spectra behavior of the common cation salts. The data are interpreted in terms of the fluctuations of the X- and Cs+ polarizabilities induced by short range ionic interactions. The Li-X pair interactions appear to be important in determining the scattered light intensity and its temperature variation. 

doi: 10.1149/199413.0284PV

G. Voyiatzis, E.A. Pavlatou, G.N. Papatheodorou, M. Bachtler, W. Freyland

The valence states and species formed by metal or electrolytic reduction of TaCl5 and NbCl5 in molten alkali chloride solvents have been determined by Raman spectroscopic measurements. In the molten solvents CsCl(l), NaCl/CsCl(eut) and LiCl/KCl(eut) the refractory metal pentachlorides MCl5 (M=Ta, Nb) were systematically reduced by adding the corresponding metal M and a series of metals L=Au, Ag, Cu, Pb, Cd, Zn and Al. Raman spectroscopic measurements of these solutions indicated that the predominant species present were the MCl6- and MCl62- octahedral having the refractory metal in the valence states five and four respectively. Equilibrium between the MCl6- and MCl62- species appear to exist in the melt solvents and no other lower valence states could be detected. In situ Raman spectroelectrochemical measurements on amorphous carbon electrodes in LiCl/KCl(eut) containing almost 5mol% TaCl5 indicated that the tetravalent TaCl6-2 species is formed on or in the vicinity of the cathode. Another tantalum chloride species with valence state lower than four was also detected from the electrode spectra.  

E.A. Pavlatou, G.N. Papatheodorou

Vibrational Raman spectroscopy has been used to identify and determine the structure of species formed in the binary mixtures ZnCl2-ACl (A=Cs, Li) and ZnCl2-ZnBr2. The data indicate that the network-like structure of the zinc halide melts breaks up with the addition of alkali metal halide and with increasing temperature. The tetrahedral structure around the Zn is preserved but one, two, three or four "terminal" halogen atoms having Aas nearest neighbors are formed. Different bridging bonding states are proposed to exist in the pure ZnCl2 glass and liquid.

doi: 10.1149/199216.0072PV

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, Tg, 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) × 10−3−1 and its dependence on the composition arises mainly from the composition-dependent Tg. 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 Tg values. The 3:2 fragile glass (Tg=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

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