Papers

E.A. Pavlatou, S.N. Yannopoulos, G.N. Papatheodorou, G. Fytas

The Journal of Physical Chemistry B (1997), 101, 8748-8755

Photon correlation spectroscopy in both polarized and depolarized geometries has been utilized to study density and orientation fluctuations in ZnCl2, ZnBr2, and their symmetric mixture in the metastable supercooled state. The pure components display behavior intermediate between strong and fragile glasses reflected in the shape (L(logτ)) and dynamics (τ) of the relaxation function C(t), being similar for both density (Cρ(t)) and orientation (C2(t)) correlation functions. In contrast to polymer blends, concentration fluctuations have no measurable broadening effect on the shape of C(t). Instead, L(logτ) becomes narrower with increasing temperature and affects differently Cρ(t) and C2(t) for the zinc halide mixture. Despite local homogeneity in the thermodynamically ideal mixture, ZnBr2 with the higher glass transition temperature (Tg=395K) was found to dominate the primary α-relaxation in the symmetric binary mixture. In the framework of "two-fluid" models, the distribution and the temperature dependence of τ support rigid microheterogeneities on the order of 2nm. The additional slow process, observed in fragile glasses so far, appears to affect also the anisotropic scattering from the pure components.

doi: 10.1021/jp970343s

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

The Journal of Chemical Physics (1996), 7, 2660-2667

Molten mixtures of lithium chloride and cesium chloride have been studied using Raman spectroscopy at temperatures up to 850°C. Reduced isotropic and anisotropic Raman relative intensities have been measured at different compositions and temperatures. The spectra of the mixtures show two bands with ‘‘characteristic’’ frequencies ωCs and ωLi close to the frequencies of the bands of the pure components. Both ωCs and ωLi shift to higher energies with increasing mole fraction of CsCl and LiCl, respectively. The scattering intensity spectra of the mixtures are compared with the simulated additive spectra of the component salts. Increasing temperature increases drastically the isotropic intensity of the ωLi band but has minor effects on the anisotropic intensities. The data are discussed and interpreted in terms of interaction‐induced polarizability fluctuations. Short range overlap interactions, mainly between Li+ and Cl, and the "symmetry" of the local structure around the anion determine the breathing‐like fluctuations which contribute to isotropic scattering and account for the drastic temperature and composition dependence of the Raman intensities. The main contribution to anisotropic scattering arises from near‐neighbor dipole‐induced‐dipole interactions between the highly polarized Cs+ and Cl ions.

doi: 10.1063/1.472129

E.A. Pavlatou

Chimica Chronica-New Series (1994), 23, 265-270

Raman spectra of ZnCl2, ZnBr2 and their mixtures were measured in the glassy and liquid state, at various temperatures and compositions. The overall behavior of the spectra indicates the presence of different bonding states in the "network" structure, where halogen atom sharing of the ZnX4 (X=Cl, Br) tetrahedral groups occurs. Furthermore, at elevated temperatures the "network" structure breaks up, creating in the melt non bridging halogen atoms which are bound to a three-fold coordinated Zn atom. The spectra of the binary ZnCl2-ZnBr2 system show that the mixtures are formed by a random closest packing of halogen atoms. The observed similarities between depolarized reduced Raman spectra and the total density of vibrational states in the glassy and liquid state are presented and discussed.

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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