Electrical Properties in Phosphate Glass Containing Li₂O, K₂O and Fe₂O₃

Abstract

The electrical conductivities of glasses in the system Li₂O-K₂O-Fe₂O₃-P₂O₅ were observed in the temperature range 20° to 160°C. Whereas alkali ions, Li₂O and K₂O, give rise to ionic conduction, Fe₂O₃ to electronic conduction. In the series (40-x) Li₂O-xK₂O-60P₂O₅ the conductivity was found to have a minimum and the activation energy to have a maximum at the composition corresponding to x=20mol%. In the series (20-x) Li₂O-xK₂O-20Fe₂O₃-60P₂O₅ the curves of logσ versus the content of K₂O were shown to have the minimum at about x=5mol%, however, the activation energy curves decreased with increasing K₂O content without having any maximum. This can be considered from the fact that a part of Li₂O combines with Fe₂O₃ in some ways, i.e. by way of producing a phase LiFeO₂ or LiFe₅O₈. Li⁺ utilized for the ionic conduction may actually decrease.
In complicated composition of glass, the simple additivity law does not hold. The experimental values of logσ_100°C in the 20Li₂O-20Fe₂O₃-60P₂O₅ glass was about one order of magnitude lower than those calculated from the formula of additive property,
σ=K₁⋅y⋅e^-ΔH_a/RT+K₂⋅(1-y)²⋅c(1-c)⋅e^-ΔH_e/RT
where K₁, K₂ are constants, ΔH_a the activation energy for the glass 40R₂O-60P₂O₅, ΔH_e the activation energy for the glass 40Fe₂O₃-60P₂O₅, y the concentration of R₂O, (1-y) the concentration of Fe₂O₃, c the ratio of Fe²⁺/Fe²⁺+Fe³⁺, R gas constant, and T absolute temperature.
On the other hand the experimental values of the conductivity in _xK₂O-(40-x)Fe₂O₃-60P₂O₅ glasses were found to be approximately equal to the calculated values from the above formula of additivity law. In this experiment, therefore, the additivity law holds only for the glass containing K₂O as alkali component. Accordingly the formula of the conductivity in the glasses containing both Li₂O and Fe₂O₃ is considered to be supplemented by a correction term owing to the producing of the phase LiFeO₂ and LiFe₅O₈ which hinder both ionic and electronic conduction; that is, we have a conduction formula,
σ=K₁⋅y⋅e^-ΔH_a/RT+K₂⋅(1-y)²⋅c(1-c)⋅e^-ΔH_e/RT+σ_c.
σ_c is a correction term.