Relationship between Pre-Exponential Factor and Activation Energy of Conductivity in Sintered Ln_9.33+x/3Si_6-xM_xO₂₆ (Ln=La, Nd, Sm, M=Al, Gd) with Apatite-like Structure

Abstract

Electrical conductivity of a series of Ln_9.33+x/3Si_6-xM_xO₂₆ (Ln=La, Nd and Sm, M=Al and Ga) sinters was examined. The pre-exponential factor and activation energy of the ionic conduction were well related by Meyer-Neldel rule, i.e., G=G₀₀ exp (ΔE/kT₀) exp (-ΔE/kT). Most of the sinters were composed with an apatite-like phase in major and sub-products such as LnSiO₅ and Ln₂O₃ in minor proportions. The single phases with apatite-like structure were obtained for the composition of x=1.5. At this composition, the activation energy increased with a decrease in the ionic radius of Ln. The lowest activation energy and highest pre-exponential factor were observed for the combination of Ln=La and M=Al. This result was interpreted by considering the formation of the 4e sites for oxygen with under occupancy and oxygen ion interstitials surrounded with six 6h(La) sites.