Diffusion of rubidium ion in silver bromide and temperature dependence of Frenkel formation energy

The diffusion of Rb⁺ ion in AgBr has been measured in the temperature range 184-421^∘{}C by a tracer and serial-sectioning technique. The temperature dependence of the diffusivity follows a linear Arrhenius law with parameters D₀=(1.04±0.30)×10⁴ cm²/sec and H=1.33±0.02 eV. When one assumes no Coulombic binding between the monovalent solute and vacancy, the activation enthalpy for the migration of an Rb⁺ ion by a vacancy mechanism, Hₘ, is estimated to be 0.75 eV. The curvature in the Arrhenius plot, expected on the basis of a temperature-dependent defect-formation energy, is not observed, perhaps owing to a temperature-dependent elastic binding energy between the oversized Rb⁺ ion and neighboring vacancy. An important trend relating a variation in Hₘ to the mismatch between the sizes of the solute and host ion for all the reported impurity-diffusion studies in AgBr has been observed and discussed.