Anionic conductivity in Na0.5-x(R,R*)0.5+xF2+2x single crystals (R,R*=RE) with the defect fluorite-type structure

The effect of the variations in the elemental composition on the electrical conductivity of the material is studied for the Na0.5-x(R(1-y)Ry(y)*)(0.5 +x)F2+2x solid solutions (x = 0.08-0.11, y = 0.002-0.100) in which the pairs of rare earth elements (R, R*) are (Y, Nd), (Y, Yb), (Lu, Nd), (Lu, Ho), (Lu, Y), and (Lu, Tm). The conductivity was measured on single crystals grown from melt by the Bridgman-Stockbarger method in a fluorinating atmosphere. The anionic conductivity of the ternary Na0.5-x(R1-yRy*)(0.5+x)F2+2x solid solutions is almost the same as the conductivity of the binary Na0.5-xR0.5+xF2+2x solid solutions. This is explained by the fact that in the transition from binary to ternary crystals, the concentration of mobile fluoride anions remains unchanged (4.5% of the total number of fluoride anions), whereas the enthalpy of conductivity (characterizing the potential barriers on the path of the fluoride-ion migration) changes only insignificantly.