Structure and electrical conductivity of compositionally complex double perovskite cobaltites

In this study, compositionally complex cobaltites with the general formula BaLnCo₂O_6-δ with three to eight different lanthanides at the Ln-site were synthesized using the solid-state reaction method and studied. Analysis of entropy metrics and configurational entropy calculations indicated that these compounds are medium entropy oxides. All of these crystallize as tetragonal double perovskites from the space group P4/mmm. The unit cell parameters are controlled by the average ionic radius, not the configurational entropy. On the other hand, the oxygen non-stoichiometry is consistently higher than in the case of low entropy double perovskite cobaltites. The total electrical conductivity of all materials in studied conditions is well above 50 S/cm, peaking at 1487 S/cm for BaLa_1/3Nd_1/3Gd_1/3Co₂O_6-δ at 300°C. The electrical conductivity decreases with the number of substituents.