Influence of Nonstoichiometry in Fluoride Phase on the Ionic Conductivity of Composite Fluoride‒Oxide Materials 90PrF3 · 10Al2O3 and 90Pr0.95Sr0.05F2.95 · 10Al2O3

Abstract

The two-phase fluoride oxide composite 90Pr_0.95Sr_0.05F_2.95 · 10Al₂O₃ (mol %) composed of a highly conductive fluoride matrix with a tysonite structure (LaF₃) and aluminium oxide is of interest as a solid electrolyte membrane in a potentiometric oxygen sensor with an auxiliary electrode. The ionic conductivity of the composite 90Pr_0.95Sr_0.05F_2.95 · 10Al₂O₃ has been determined by impedance spectroscopy in vacuum (~0.1 Pa) in the temperature range of 293‒700 K. Its dominant component is the conducting non-stoichiometric tysonite phase Pr_0.95Sr_0.05F_2.95. To estimate the contribution of this phase to the composite conductivity, the conductivities of the single-phase (single-crystal) PrF₃ and Pr_0.95Sr_0.05F_2.95 bases and a twophase composite 90PrF₃ · 10Al₂O₃ with a stoichiometric fluoride matrix PrF₃ have been investigated. The use of the nonstoichiometric phase Pr_0.95Sr_0.05F_2.95 as a composite matrix instead of PrF₃ leads to an improvement of ionic conductivity by a factor of ~200: to 5 × 10^–3 S/cm at 400°C.