David L. Sidebottom

Fundamental questions relating to ion conduction in disordered solids

A number of basic scientific questions relating to ion conduction in homogeneously disordered solids are discussed. The questions deal with how to define the mobile ion density, what can be learnt from electrode effects, what the ion transport mechanism is, the role of dimensionality and what the origins of the mixed-alkali effect, the time-temperature superposition, and the nearly constant loss are. Answers are suggested to some of these questions, but the main purpose of the paper is to draw attention to the fact that this field of research still presents several fundamental challenges.

Comparison of KWW and power law analyses of an ion-conducting glass

Abstract Measurements of the electrical impedance of LiPO3 glass at frequencies between 1 Hz and 106 Hz were analyzed using two popular procedures: (1) fitting the conductivity and permittivity to power laws in frequency, and (2) fitting the electrical modulus to a stretched exponential. The parameter, β, which expresses the non-exponential nature of the relaxation of electrical perturbations, is substantially different for these two procedures. The value obtained from the frequency dependence of the conductivity via the coupling model, βn, is similar to that seen for mechanical and nuclear magnetic resonance relaxations, suggesting that it may be more physically meaningful, whereas that from the electrical modulus, βm, is significantly larger. This dissimilarity is surprising since others often use these two βs interchangeably when testing theoretical predictions. This result will significantly after previous conclusions drawn from such tests.

Structure and optical properties of rare earth-doped zinc oxyhalide tellurite glasses

Zinc tellurite glasses appear to be excellent candidates for hosting rare earth ions since they provide a low phonon energy environment to minimize non-radiative losses as well as possess good chemical durability and optical properties. The optical behavior of the rare earth ion can be manipulated by modifying its local environment in the glass host. The authors report measurements of the emission lifetime, optical absorption, and vibrational density of states of the glass system (ZnO){sub x}(ZnF{sub 2}){sub y}(TeO{sub 2}){sub 1{minus}x{minus}y}doped (0.1 mol%) with a series of rare earths. Phonon sideband spectroscopy has been successfully employed to probe vibrational structure in the immediate vicinity of the rare earth ion. The authors observe a significant increase in the emission lifetime (from approximately 150 {mu}s to 250 {mu}s) of Nd{sup 3+} with increasing fluorine substitution.

Scaling Behavior in the Conductivity of Alkali Oxide Glasses, Polymers, and Doped Crystals

Abstract Although the frequency dependent conductivity, σ(ω), of ion-containing glasses displays power law dispersion (σ(ω) ≈ ω″) that can usually be described by a master curve, several findings have suggested that this scaling fails at low temperatures as indicated by a temperature dependence of the scaling exponent, n . This behavior is investigated in the frequency range between 1 Hz and 10 6 Hz for different materials including alkali metaphophate glasses and a polymer. Two distinct regimes of conductive behavior, σ I and σ II , are identified. The first, σ I , is strongly temperature dependent and appears to obey a master curve representation. The second, σ II , exhibits only a weak temperature dependence with a roughly linear frequency dependence. A strong depression of σ I occurs for the mixed alkali case, but σ II is unaffected and occurs at roughly the same location in all the alkali compositions studied. It is proposed that σ II does not arise from cation motion, but rather originates from a second mechanism likely involving small distortions of the underlying glassy matrix. This assignment of σ II is further supported by the approximately universal location of σ II , to within an order of magnitude, of a variety of materials, including a polymer electrolyte and doped crystal. Since σ I ( T ) and σ II ( T ≈ const.) are viewed as separate phenomena, the temperature dependence of the scaling exponent is shown to result merely from a superposition of these two contributions and does not indicate any intrinsic failure of the scaling property of σ I .

Increased radiative lifetime of rare earth-doped zinc oxyhalide tellurite glasses

We have investigated the structural and optical properties of rare earth-doped zinc tellurite glasses modified by the substitution of ZnF2. Raman and phonon sideband spectroscopies were employed to characterize changes in the glass structure as well as to probe vibrational behavior in the immediate vicinity of the rare earth ion. These measurements are combined with photoluminescence and optical absorption to monitor the effect of halide substitution upon the optical behavior of the rare earth dopant. A substantial increase in the intrinsic radiative lifetime of Nd3+ is observed with increasing halide concentration.

Structural correlations in the ac conductivity of ion-containing glasses

Abstract The power law dispersion of the conductivity in most ion-containing oxide glasses appears to be universal. This universality indicates that no correspondence exists between the degree of non-Debye behavior and the microscopic structure of a glass host. Despite its universal frequency dependence, we demonstrate how the ac conductivity defines a special length scale, L , that represents average diffusional displacement of an ion per activated jump and which can be correlated to properties of the glass host. We compare this length scale to the inter-ionic spacing and demonstrate how both increase with decreasing ion concentration. Although there is much evidence for universality, we suggest that the conductivity exponent may in fact be influenced by the ‘dimensionality’ of the conduction process.

Light‐scattering studies of a ternary mixture: Comparison of field variables for critical phenomena description

We present static turbidity and dynamic light‐scattering measurements near the critical double point of a mixture of secondary butyl alcohol, tertiary butyl alcohol, and water. At the critical double point of this mixture an upper two‐phase miscibility loop merges with a lower two‐phase region. When temperature is used as a field variable to describe the critical behavior of the turbidity and correlation length of the mixture, nonuniversal behavior is seen in both the critical exponents and amplitudes. If, however, the tBA concentration difference from the critical curve is used, universality is found in both exponents and amplitudes. We thus term this concentration difference to be the relevant parameter for proper description of the critical behavior and, following Griffiths and Wheeler [Phys. Rev. A 2, 1047 (1970)], show this relevant parameter was determined by it never having tangential paths of approach to the critical curve.

Dynamic light scattering study of nonexponential relaxation in supercooled 2Ca(NO3)2⋅3KNO3

We have performed depolarized dynamic light scattering (photon correlation) measurements on the ionic glass forming mixture 2Ca(NO3)2⋅3KNO3 above its glass transition in the temperature range 68–86 °C. We find the relaxation function is a stretched exponential. The average relaxation time follows an Arrhenius temperature dependence and can be related to motions on a molecular scale. The width parameter β does not vary with temperature and is β=0.40±0.05. Combination of our β data with other literature values shows that β evolves with temperature over a larger range. At high temperature, β crosses over from β=1 to β<1 at a temperature which matches well with where the viscosity first deviates from a power law behavior. β then evolves downward with lowering temperature and then settles at 0.4 roughly 30 °C above the glass transition Tg. This behavior corresponds well with recent theory relating the glass transition to the ramification of phase space but it implies that the percolation of phase space occurs ...

The importance of ion-polymer crosslinks in polymer electrolytes

Abstract Raman-, Brillouin-and photon correlation spectroscopy have been used to study LiCF 3 SO 3 and NaCF 3 SO 3 containing polyethers (PEO and PPO). At focus are factors controlling the ion-polymer interactions, the formation of transient ionic crosslinks and their effect on the local flexibility. Brillouin and photon correlation spectroscopy show that the structural relaxation time is independent of the polymeric chain length for chains longer than n≈20 repeat units. For shorter chains OH bridging slightly stiffens the dynamics whereas for CH 3 end-capped chains the local chain flexibility increases dramatically as n decreases. The crossover at ∼20 monomer indicates the number of monomers involved in the cooperative segmental rearrangements needed for structural relaxations in high M w complexes, and thus also for an ionic displacement to occur in these systems. It is found that transient crosslinks are established for n > 4 and they considerably slow down the local dynamics. The density of crosslinks is found to be strongly temperature dependent and this is the reason for departure from T g scaled dynamics in the complexes.

Brillouin scattering in alkali metaphosphate glasses and melts

Abstract We report the results of Brillouin scattering measurements performed on a series of alkali metaphosphate glasses and melts including the mixed alkali composition. Using independent viscosity data, we have applied a time–temperature superposition approach to obtain from the relevant features of the Brillouin spectra the relaxation characteristics of the complex longitudinal elastic modulus. A mixed alkali effect is evidenced in the Brillouin scattering which occurs in the gigahertz range. We find a relaxation process that can be described by a Kohlrausch function with β ≈0.30 for single alkali compositions, but is narrower ( β ≈0.40) at the mixed alkali composition.