John A. Duffy

A review of optical basicity and its applications to oxidic systems

Abstract A description is given of the theoretical background to the optical basicity concept, which is based on orbital expansion effects reflecting the nature of the chemical bonding between a Lewis acid-base pair. Optical basicity is particularly useful for oxidic network systems, such as silicate, phosphate, or borate in crystalline, vitreous, and liquid (molten) form. The experimental measurement of optical basicity, from the ultraviolet spectra of probe ions such as Tl+ and Pb2+, is described, and it is shown how optical basicity values can be assigned to individual oxides such as Na2O, CaO, or SiO2. These make it possible to calculate a “theoretical” optical basicity value, Λth, from chemical constitution, thereby allowing correlations to be made with various properties of, for example, molten silicates as a function of composition. The correlations have important applications for redox behaviour, e.g., in glass making and in the performance of metallurgical slags, and should find application in geochemistry, for example, to Fe 2 + Fe 3+ and other redox ratios in magmas. Furthermore, in the chemical reactions between metal silicate, borates, etc., the Λth values indicate how minimisation or elimination of optical basicity differences between the reactants is a major driving force.

A common optical basicity scale for oxide and fluoride glasses

Abstract Arguments are presented justifying the use of fluoride optical basicity values (e.g. λ ( BaF 2 ) = 0.50, λ ( SiF 4 ) = 0.21, λ ( ZrF 4 ) = 0.38) for calculating the optical basicity of all-fluoride and oxide/fluoride glasses. The resulting λ values agree with those obtained by probe ion spectroscopy, and provide a means for calculating trends in ideal optical basicity for glass systems such as BaF 2 ZrF 4 or AlF 3 Al(PO 3 ) 3 .

Conductivity anomalies in tungstate-phosphate glasses: evidence for an ion-polaron interaction?

Abstract Conductivity anomalies are observed in mixed ion-polaron conducting glasses in the system Li 2 O: WO 3 : P 2 O 5 . Deep minima in conductivity (reminiscent of the mixed alkali effect) occur when small amounts of Li 2 O are introduced into WO 3 − y : P 2 O 5 glasses. By contrast, glasses melted in air are always deep blue in colour, except when large amounts of Li 2 O are incorporated. It is suggested that negatively charged polarons (effectively the d-electron located on a W 5 + centre) interact strongly with mobile cations to form uncharged diffusing entities. This may be a general phenomenon in mixed ionic electronic conductors where mobilities of ions and electrons are comparable in magnitude.

Acid–base properties of molten oxides and metallurgical slags

The application of the Lux–Flood concept to acid–base reactions in oxide systems is discussed in the light of spectroscopic information concerning the “state” of oxide ions. In slag systems (e.g. molten silicates) it is possible for the basicity to be expressed in terms of electron donor power as an optical basicity (Λ), and not in terms of the activity of O2– ions. CaO is chosen as the reference oxide of unit basicity; Λ-values are either measured spectroscopically or calculated from the equation Λ=ΣXiΛi, where Xi and Λi are equivalent fractions and (assigned) basicity values for slag components. The optical basicity scale provides a means for comparing acid–base indicators in oxide melts, rather analogous to the use of the Hammett acidity function (HO) in strong protonic acids and is applicable to multicomponent systems. Possible applications of optical basicity to metallurgy are exemplified by reference to the sulphide capacities (Cs) of iron and steelmaking slags. Data for ≈ 130 blast furnace slags show a good correlation between log Cs and Λ, even for slags which do not contain SiO2. Evidence is supplied that optical basicities can also be derived from ESCA measurements; Λ-values obtained in this way might be useful in assessing the properties of steel making slags.

The refractivity and optical basicity of glass

Abstract The refractivity of the oxide ion, RO2−, in alkali and alkaline-earth silicate glasses is shown to vary linearly with the optical basicity calculated from the basicity moderating parameters of the constituent cations. Deviations in the relationship are almost eliminated by adjusting the refractivity values of the cations. The relationship allows the basicity moderating parameters to be estimated for cations with previously unassigned values, for example, Pb2+ in lead silicate glasses and Zr4+ in zirconium silicate glasses. Although the relationship appears to extend satisfactorily to ternary glass systems, e.g. K2OAl2O3SiO2, it becomes less straightforward when structural changes occur in the glass, for instance in the sodium borate system. These structural changes are often detected by the optical basicity trends signalled by probe ions, and it is suggested that refractivity measurements could indicate areas for which investigation by probe ion spectroscopy is profitable.

Lithium borate glasses: a quantitative study of strength and fragility

Abstract Methods for quantifying the strength/fragility of glass-forming systems are described, with particular reference to the alkali borates. One parameter used is the limiting slope of the log viscosity versus T ∗ /T plot as T approaches T ∗ , the temperature at which the viscosity is 1012 Pa s. A comparison is made with ΔCp/Cp1, the normalised change in specific heat capacity at Tg, as an alternative measure of the degree of fragility in these glass-forming systems. Both approaches indicate that increasing alkali oxide content results in increasing fragility within the borate system, although anomalous correlations can result when different glassy system are compared. The extent to which the limiting viscosity slope and ΔCp/Cp1 probe different aspects of structural change and fragility is briefly explored.

Chronoamperometric response of the cell ITO|HxWO3|PEO-H3PO4(MeCN)|ITO

Electrochromic devices of the general type indium tin oxide (ITO)|WO3|polyethylenc-oxide (PEO)-H3PO4|(H)ITO were constructed. The mechanism of electrocoloration involves protons decanting from the pre-intercalated ITO to form the cell ITO|HxWO3|PEO-H3PO4|ITO. During potentiostatic coloration, the current goes through a minimum and a maximum at short times. Simultaneous monitoring of cell transmittance shows that the processes causing the current peak are colour forming in origin. It is suggested that the peak is caused by an electronic mobility threshold.

The effect of moisture on tungsten oxide electrochromism in polymer electrolyte devices

Abstract Cells containing polymer electrolyte, plasticised with acetonitrile, ITO/WO3/PEO-H3PO4 (AN)/ (H) ITO, were assembled using both dry and damp electrolyte. Electron micrographs taken on damp cells showed extensive changes within the WO3 layer leading finally to disintegration. The WO3 film first lost its electrochromic memory. These changes in cell electrochemistry were monitored by impedance spectroscopy and mechanisms for self bleaching were discussed.

Analysis and prediction of acid–base reactions between oxides and oxysalts using the optical basicity concept

The chemical reactions of (i) oxides with each other, e.g. CaO with SiO2, (ii) oxides with oxysalts, e.g. CaO with CaSiO3, and (iii) oxysalts with each other, e.g. Ca2SiO4 with Ca3(PO4)2, have been considered from the point of view of acid–base theory. From known optical basicity values for alkali and alkaline-earth metal oxides and also for MgO, Al2O3, B2O3, SiO2, P2O5, and SO3, optical basicities were calculated for a large number of oxysalts. Examination of existing data on the reactions of these oxidic compounds at elevated temperatures, in the form of sub-solidus compatibility diagrams for ternary systems, has revealed that generally the optical basicity spanned by the products is less than that spanned by the reactants. This indicates that acid–base neutralisation is the major driving force behind the reaction; occasionally this principle is violated, as when other factors such as a stable network structure are of prime importance. It is shown how optical basicity analysis offers a method of constructing the tie-lines of sub-solidus compatibility diagrams, and it is suggested that this might be used as a guide for checking uncertainties in such diagrams.

Electrochromic display devices of tungstic oxide containing vanadium oxide or cadmium sulphide as a light-sensitive layer

Solid-state electrochromic display devices (ECDs) have been prepared using vacuum deposited tungstic oxide as the primary electrochromic material. Such devices form colour during irradiation from a fluorescent-tube as light source. Initial experiments incorporated (evaporated) CdS films and the proton-conducting poly(ethylene oxide)-phosphoric acid electrolyte (1:1) plasticised with acetonitrile. Successful photoelectrochromic devices were prepared in this way provided the electrolyte was anhydrous, since films of CdS are sensitive to water-attack. Another device to evince photoelectrochromism was one containing layers of WO3 and V2O5. This combination of oxides was the most promising, responding to a 1/20s flash of intense white light. As yet, the colour formed in this way cannot be electro-bleached. These displays may also be used as normal—electrolytically driven—devices in the absence of irradiation.