Proton transport in doped La-metaphosphate glasses

Abstract

The electrical properties of [La (1 − x) M x ]–P 3O 9 metaphosphate glasses, where M is Ba, Sr, Ca, and 0 ≤ x ≤ 0.8 were investigated in the 300–500 °C range. The protonic conductivity increases two orders of magnitude from the unsubstituted to the 60% substituted glass reaching a maximum conductivity of about 10 − 5 S/cm at 450 °C. The activation energy is found to be approximately independent of concentration though dependent on the type of modifying substitutional cation, and decreases from about 1.04 eV for Ca and Sr to ∼ 0.96 eV for Ba. AC impedance measurements were used to find the DC ionic conductivities. Mean ion travel distances at the frequencies, ν Ζ * or ν Μ *, associated with the maximum in Z″ or M″ were found to be up to 30 times larger than the average oxygen spacing in the glasses. While diffusion coefficients derived from conductivities, D σ , were vastly lower than an uncorrelated diffusion coefficient computed from first principles, the travel distance at ν Z * or ν Z * derived from a simple random walk expression using D Γ , was surprisingly close to those derived from the more rigorous statistical treatments. The ratio ξ = D σ / D Γ ≪ 1 may be interpreted as indicative of highly correlated ion motion and strongly reduced mean attempt frequencies, together with a mean activation energy for motion increased by Δ E = E σ − E Γ over that of the uncorrelated intertetrahedral motion of an ideal crystal characterized by D Γ .