Evangelia A. Pavlatou

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 MCl₆^2- octahedral having the refractory metal in the valence states five and four respectively. Equilibrium between the MCl₆^- and MCl₆^2- 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 A⁺ as 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, 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