Effect of Pb Substitution on Electrical Conduction and Sinterability of LaPO₄-Based Protonic Conductor

Abstract

In this study, we investigated sinterabilities and electrical conductivities at 550-900°C of (La_0.97M_0.03)PO₄ (M = Ca, Sr, and Pb) with a monazite structure. It was found that the Pb substitution could improve the sinterability of LaPO₄ even at low sintering temperature. Although the Pb-substituted sample exhibited hole conduction at the high-temperature range, protonic conduction became to be observed with decreasing temperature. Compared with the Sr-substituted sample, (La_0.97Pb_0.03)PO₄ showed lower proton conductivity. Taking these results into account, we performed a co-substitution of Sr and Pb for LaPO₄ to enhance both the protonic conduction and the sinterability. As a result, it was demonstrated that (La_0.94Sr_0.03Pb_0.03)PO₄ had a higher relative density than Sr-substituted ones, and the co-substitution could improve the protonic conduction. It can be considered that the co-substitution is a promising way to prepare dense LaPO₄-based protonic conductors.