Relationship between pre-exponential factor and activation energy of conductivity in sintered Ln9.33+x/3Si6-xM xO26 (Ln = La, Nd, Sm, M = Al, Gd) with apatite-like structure

Abstract

Electrical conductivity of a series of Ln9.33+x/3Si 6-xMxO26 (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-Nel-del rule, i.e., G = G 00 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.