Conductivity enhancement in fluorite-structured Ba 1−xLa xF 2+x solid solutions

Abstract

The ionic conductivity of single crystals of the fluorite-structured solid solutions Ba 1− x La x F 2+ x (10 −3 <×<0.45) has been studied as a function of temperature and composition in the range 300–900 K. Three regions can be discerned in the concentration dependence of the ionic conductivity: a dilute concentration region ( x<10 −3), where classic relations between solute content and ionic conductivity hold; an intermediate concentration region (10 −3< x≲5×10 −2), where large changes occur in the conductivity activation enthalpy and the magnitude of the conductivity; and a concentrated solid solution region ( x≳5×10 −2) characterized by enhanced ionic motion. In the dilute region the migration enthalpy for interstitial fluoride ions is determined to be 0.714 eV, while a value of 0.39 eV is found for the (La BaF i) X association enthalpy. The defect chemistry in the intermediate concentration region is shown to be controlled by a superlinear increase of the concentration of mobile defects, while in the concentrated solid solution region a composition-independent amount of ≈1 mole% of interstitial fluoride ions with enhanced mobility, carry the current.