Extensive research of conductivity in the fluorite-like KLn₄Mo₃O₁₅F (Ln = La, Pr, Nd) rare earth molybdates: theoretical and experimental data

The conductive properties of fluorite-like structures KLn(4)Mo(3)O(15)F (Ln = La, Pr, Nd: KLM, KPM, KNM) have been studied theoretically and experimentally. Theoretical studies included the geometrical-topological analysis of voids and channels available for migration of working ions; bond valence site energy calculations of the oxygen ions' migration energy; quantum-chemical calculations for the estimation of the oxygen vacancies formation energy. Experimental measurements of conductivity were made using impedance spectroscopy and as a function of oxygen partial pressure. The total conductivity was similar to 10(-3) S cm(-1) for KLM and similar to 10(-2) S cm(-1) for KPM and KNM at 800 degrees C. Measurements with changes in partial pressure proved the mixed nature of electric transport in KLM, KPM, and KNM phases, with predominantly ionic conductivity. The measured ion transference numbers in air reached approximately 0.9 at 800 degrees C for the KPM and KNM ceramics. Also, evaluated proton transfer numbers were less than 10%, indicating a small contribution to the total conductivity.