Internal friction and dielectric losses of mixed alkali borate glasses

The internal friction and dielectric losses of NaK, LiNa, LiCs, LiK, NaCs, KCs, AgLi, AgNa, AgK and AgCs borate glasses were measured as functions of the temperature at various frequencies. In general, the behavior of mixed alkali borate glasses is very similar to the behavior of the comparable phosphate and silicate glasses. The magnitude of the mixed alkali peak was found to vary systematically with the size of the involved alkali ions. The silver-containing glasses also show the mixed alkali effect. The borate glasses are briefly compared with the silicate and the phosphate glasses and their behavior is found to be in agreement with the recent proposal that the mixed alkali peak is caused by an electro-mechanical cross effect.     

Internal friction of mixed alkali metaphosphate glasses: (II). Discussion

The internal friction and dielectric losses of mixed LiNa, LiK, LiCs, LiAg, NaK, NaCs and NaAg metaphosphate glasses are interpreted on the basis of explanation proposed for the mixed alkali effect. It was found that the magnitude of the mixed peak correlates better with size differences than with mass differences. The intermediate temperature peak is treated as a mixed proton-alkali peak. The large mechanical loss peak, appearing when the alkalis are mixed, was attributed to a coupled movement of dissimilar alkali ions and an explanation of the nature of this coupling is proposed. From this model it is predicted that mixed peaks can occur in any electrically insulating material containing dissimilar charge carriers.     

Relaxation processes in mixed alkali NaK methaphosphate glasses

The internal friction and the dielectric losses of NaK metaphosphate glasses have been investigated. A single alkali and a mixed alkali internal friction peak were observed. The single alkali peak shifted to higher temperatures with increasing concentration of the second alkali and eventually disappeared. The activation energy of this peak increased with the alkali mixing ratio. As in silicate and borate glasses, the single alkali peak correlates closely with the dielectric loss. The mixed peak showed a dramatic increase in size with the addition of the second alkali, but the activation energy was practically independent of the alkali mixing ratio. The large reduction in height of the mixed peak, observed on annealing, is discussed together with the influence of water.     

The mixed alkali effect in the Na2OCs2OSiO2 glasses

As an approach to the mixed alkali effect in glass, the self-diffusion coefficients of sodium and cesium ions in Na₂OCs₂OSiO₂ glasses were measured at temperatures 350–550°C. Electrical conductivity of the glasses and the transport number for sodium ions were also measured. The substitution of the alkali ions in the glass by different alkali ions caused the mobility of each alkali ion to decrease pronouncedly and the activation energy for migration to increase rapidly. The increase of activation energy was attributed to an increase in alkali-oxygen bond strength resulting from the presence of two kinds of alkali ions. This is related to the expectation that the activity of the alkali ions decreases when two alkali ions are mixed.     

Structure and properties of silver phosphate glasses — Infrared and visible spectra

The infrared and visible spectra of glasses in the Ag₂OP₂O₅ glass-forming system were obtained. The infrared data were interpreted as indicating the presence of polymeric chains in these glasses. The partial covalent nature of the AgOP bond was discussed. A mixed NaPO₃AgPO₃ glass showed no unexpected bands in the infrared spectrum, again showing that silver is behaving in a manner similar to alkali metal ions in phosphate glasses. The shift in the absorption edge in the visible spectra of glasses of different Ag/P ratio was shown to arise from either an increase in the concentration of nonbridging oxygens with increasing silver content, or the presence of colloidal silver metal particles.     

Bond character and properties of the mixed alkali system Na2OK2OAl2O3SiO2

The mixed alkali glass system Na₂OK₂OAl₂O₃SiO₂ was investigated. Density, transformation temperature, refractive index, and chemical durability were studied. Optical absorption and ESR spectra of the CuO-doped glasses were determined.Calculations of the polarizability of O^2?, bonding parameters of the Cu²⁺ complex, and the packing density are presented. It was found that for the mixed alkali glasses, the oxygen- alkali bond has a more ionic character than expected from additivity. This fact enables the non-linear changes of the refractive index, of the shift of the Cu²⁺ absorption band, and of the covalency to be interpreted as the Na mole fraction is varied. It is also possible to explain qualitatively the density, T_g and chemical durability non-linear variations with change of the Na content by the ionicity deviations of the bond character and the postulated pairs of Na⁺ and K⁺ ions in the mixed alkali glasses.     

Tracer diffusion and electrical conductivity in sodium-cesium silicate glasses

The tracer diffusion coefficients of ²²Na and ¹³⁷Cs, and the electrical conductivity have been measured in the (Na, Cs)₂O:3SiO₂ glasses as a function of temperature and Cs/Na ratio. Complex impedance analysis was used for the conductivity measurements. The Haven ratio at 396.5°C increases from 0.3–0.4 in single-alkali glasses to 0.8 for the mixed-alkali compositions. The results are explained in terms of a single-jump mechanism; interactions between alkali ions and non-bridging oxygen ions, and between different alkali ions, produce the observed correlation effects.     

The mixed alkali effect in lithium-sodium borate glasses

The electrical conductivity of a series of 0.35 (Li, Na)₂O·B₂O₃ glasses shows a minimum at the composition Na/(Na+Li)～0.6, which becomes stronger as the temperature is decreased; the activation enthalpy for electrical conductivity shows a maximum at this composition. In general, replacing 1% of the total oxygen concentration by chlorine or bromine (keeping the total alkali content fixed) in these glasses increases the conductivity; fluorine doping has an opposite effect. The mixed alkali effect, expressed in terms of the compositional dependence of the activation enthalpy for conductivity, is enhanced when borate glass is doped with fluorine, but is slightly diminished when doped with chlorine or bromine. The results are explained in terms of the structure of halogenated alkali-borate glasses, and discussed in relation to the origin of the mixed alkali effect.     

Mixed alkali effect on Vickers hardness and cracking

Vickers indentation measurements were carried out on borate, silicate and aluminophosphate glasses, each series comprised of samples of different relative alkali ratios (Na/Na + Li). All the glass series exhibited nonlinear variations of hardness with relative alkali ratio, which was attributed to the reduced plastic flow of mixed alkali glasses. The mixed alkali effect was also present in the length of radial cracks although less strongly than in hardness. Using a semi-empirical model, the variations of residual stresses and fracture toughness were estimated.     

Dielectric relaxation in vitreous metaphosphates at medium temperatures

Dielectric relaxation experiments were carried out on mixed alkali metaphosphate glasses between room temperature and about 300°C (f = 10³ s^?1). The observed relaxations appeared to be connected with the movement of the alkali ions from site to site. By using stainless steel electrodes it was possible to reveal a relaxation peak free from conduction losses and due only to the migration from site to site of the cations. When two cations of dissimilar sizes are simultaneously present a mixed alkali effect was observed, i.e. an unproportionally large reduction in the alkali ion mobilities. It is shown that this effect can be at least partly explained by considering the changes in the potential energy states upon mixing dissimilar cations due to polarizing forces and coulombic interactions.     

X-ray diffraction studies of glasses in the systems Na2OMeOSiO2 (MeMg,Zn, Ca,Ba)

X-ray diffraction studies of glasses in the following ternary systems have been made: Na₂OMgOSiO₂, Na₂OZnOSiO₂, Na₂OCaOSiO₂ and Na₂OBaOSiO₂. The following heavy atom substitutions have been used: Ag for Na and Ge for Si. The changes in the electron radial distribution curves resulting from AgNa replacement can be explained as amplifications of relatively well-defined NaSi distances, which are nearly the same in all the glasses investigated. The GeSi substitution causes changes which can be explained on the basis of isostructural GeSi substitutions.     

Alkali diffusion and electrical conductivity in sodium borate glasses

The diffusion coefficient of sodium, ²²Na, and silver, ¹¹⁰Ag, and electrical conductivity for sodium borate glasses (4–24 mol% Na₂O) has been measured from 100°C to slightly below the glass transition temperature, T_g. Unlike silicate glasses where the self-diffusion coefficient is much larger than the impurity diffusion coefficient, D_Na and D_Ag had close to the same magnitude in the sodium borate glasses. In some glasses, D_Ag was slightly larger than D_Na. Na₂O-Ag₂O-B₂O₃ glasses show a relatively small mixed alkali effect, despite the significant mass difference between Ag(≈108) and Na(≈23). It is concluded that the mixed alkali effect is more dependent upon the difference in ionic radii rather than differences in mass.     

Mössbauer effect studies of alkali borate glasses

Mössbauer studies of a large number of glass samples prepared with alkali oxides in the region of glass formation are reported. Some representative samples are studied in the temperature range 85–500 K. The following glass systems are studied.

Room-temperature isomer shift values decrease gradually with the addition of alkali oxide and the values fall sharply for alkali content higher than 22 mol%. We conclude that iron is in the ferric state and that the oxygen polyhedra of iron changes from octahedral to tetrahedral. The structural change results from the fact that the alkali introduces an extra non-bridging oxygen ion. The size of the alkali cation introduced into the glass has considerable influence on the isomer shift values of iron. In fact, the polarizing power decreases in the order LiNaK, hence the s-character of the FeO bond increases in the order LiNaK. Addition of Al₂O₃ has no effect on the isomer shift values of iron, showing that aluminium ions occupy network-forming positions.For some representative samples the second-order Doppler effect was studied as a function of temperature in the range 85–500 K. The thermal red shifts due to second-order Doppler effect are used to estimate the local specific heats of ferric ion in the glass system. The quadrupole splitting has weak temperature dependence, showing that Fe³⁺ is in a high-spin state.     

The influence of dissolved heavy water on the internal friction of lithium metaphosphate glasses containing 1% potassium metaphosphate

A batch of very dry lithium metaphosphate glass, containing 1% potassium metaphosphate, was prepared and dissolved completely in nearly pure heavy water. From this solution, a series of glasses with varying deuterium oxide content was prepared. The internal friction and dielectric loss data were taken. The amount of dissolved deuterium oxide was determined from the weight-loss of specimens during heating in vacuum, and checked by means of tritium labeling. It is found that dissolved deuterium oxide influences the internal friction and dielectric losses, in the same was as additions of dissimilar alkali ions do (mixed alkali effect) so that one can say that deuterium behaves similarly to alkali ions.A comparison of the present data with the results of a recent study on the influence of protons on the mechanical and electric properties in an identical host glass illustrates clearly that the mixed alkali effect is not governed by mass differences of the cations.     

Effect of alkali metal oxides on the properties of radio-photoluminescence glasses

Silver-doped Li, Na and K aluminophosphate glasses and the corresponding basic glasses were prepared by traditional melting–quenching method. The influence of alkali metal oxides on the dosimetric properties was investigated under conditions of the same dosage and fixed silver concentration. The energy dependence, sensitivity and build-up time are related to alkali metal oxides. The atomic number, ionic radius and field strength of alkali ions have important effect on these dosimetric properties. Li aluminophosphate glass possesses the lowest fluorescence response and energy dependence; Na aluminophosphate glass owns the shortest build-up time; K aluminophosphate glass has the strongest fluorescence response, longest build-up time and highest energy dependence.     

Structure of single and mixed alkali Li–Rb borate glasses by neutron diffraction

We have compared the local structure of Li–, Rb– and Li–Rb diborate glasses using neutron diffraction and reverse Monte Carlo simulations. The fraction of tetrahedral boron decreases as the Rb content increases, but is similar in the single and mixed alkali glasses at diborate compositions. Rb borate glasses exhibit an extensive cation ordering with Rb–Rb correlations at 4.7 and 7 Å. The environment of alkalis is slightly modified between the single and mixed glasses while their relative distribution is essentially random. This indicates that the alkali environment is affected by the presence of a different alkali and this cation–cation interaction could be a fundamental mechanism, together with the site mismatch, to explain the mixed alkali effect.     

Electronic structure and properties of oxide glasses (II) Basicity measured by copper(II) ion probe in Na2OP2O5, K2OB2O3 and K2SO4ZnSO4 glasses

The correlation between the basicity of oxygens measured by the Cu(II) ion probe and the non-bonding electron density on oxygens in alkali borate glasses was considered. The basicity was measured for K₂OB₂O₃, Na₂OP₂O₅ and K₂SO₄ZnSO₄ glasses and categorized into two types, δ and π, according to the symmetry property of the bonding between a Cu(II) ion and oxygen. The π basicity for borate and phosphate glasses showed an abrupt increase in the vicinity of 17 and 50 mol% alkali oxide, respectively. The values of π-type basicity varied with the composition of glass, being larger in the order: sulfate < phosphate ? borate, whereas δ basicity was constant irrespective of the glass composition. Such a change of the basicity with the composition of glass was interpreted in terms of behavior of non-bonding levels of the ligand oxygens in a glass network.     

Correlation effects on alkali ion diffusion in binary alkali oxide glasses

The correlation factor in the Nernst-Einstei equation for low sodium Na₂OGeO₂ glass determined from dc conductivity and ²²Na diffusion coefficient measurements was found to be near unity. Values of the correlation factor were also compiled from the literature for higher alkali content germanate glasses as well as for sodium borate and alkali silicate glasses. In all three systems the correlation factor was found to depend primarily on the alkali content in the glass. Specifically, uncorrelated ionic diffusion ($\text{? ? 1}$) occurs in low alkali glasses while correlated motion ($\text{? < 1}$) takes place at higher alkali concentrations. This observation is consistent with the theory that many “holes” exist in low alkali glasses through which the diffusing cation can randomly jump.     

The mixed alkali effect in sodium and potassium galliosilicate glasses II. DC electrical conductivity

The DC electrical conductivities of several series of mixed alkali galliosilicate glasses have been measured. The appearance of a minimum in the electrical conductivity of these glasses, independent of the gallium content, suggests that the mixed alkali effect is independent of the non-bridging oxygen content. These results are discussed in terms of current theories proposed to explain this anomalous behaviour.     

Mixed alkali glass spectra and structure

The far infrared and Raman spectra of several series of mixed alkali metaphosphate glasses have been investigated as a function of the mole fraction x of the network-modifying ionic oxides in xM₂O(1?x)M₂′O · P₂O₅. The frequencies of the cation-motion bands in the far infrared spectra, which correspond to cationsite vibrations, do not shift with x, indicating that the vibrationally significant local geometry and forces associated with a particular cation are unaffected by the introduction of the second cation into the glass structure. Each Raman-active band due to vibrations of the metaphosphate network occurs at a different frequency for each pure glass (x = 0 or 1), but for mixed alkali glasses only one band occurs for each type of mode and it varies linearly with x. This indicates that the cations in these mixed alkali glasses are homogeneously distributed, there is no significant molecular-level domain formation and the phosphate chains are associated with an averaged cation environment whose effect on the chain modes varies with x. A simple vibrational model is presented which shows that the cation-dependent shifts are due to small changes in network bond angles and variation of the cationsite forces.