Protonic and Native Conduction in Sr‐Substituted LaPO₄ Studied by Thermoelectric Power Measurements

Measurements of the thermoelectric power were applied to investigate the electrical conductivity in 5 mol % Sr‐substituted as a function of temperature (600 to 900°C), water vapor partial pressure (0.2 to 6 kPa), and oxygen partial pressure (4 to 100 kPa). Expressions of the thermoelectric power for materials conducting protons, oxide ions and electronic defects were derived, based on a thermodynamic treatment of entropy production by heat and charge transfer. The experimental data for 5 mol % Sr‐substituted were interpreted in terms of protonic conduction, and some additional conduction, possibly by electron holes. In air, the proton transport number is unity at 600°C when , and at 700°C when . In wet atmospheres, the transported entropy of protons in this material was found to be nearly constant, at 600 to 800°C in wet atmospheres.