Composition and Crystal Structure Effects on the Conductivity and Catalytic Activity of Rare-Earth Tungstates Ln2WO6

Rare-earth tungstates Ln2WO6 (Ln = Eu, Gd, Tb, Dy) and their high-entropy analogues were synthesized by mechanical activation of oxides. For the first time, an orthorhombic α-modification of Dy2WO6 (sp. gr. Pm21n) was obtained and characterized. It was demonstrated that the replacement of a single rare-earth element with a combination of five cations (La, Nd, Gd, Tm, Y) leads to the stabilization of the tetragonal phase (sp. gr. P4̅21m) of the high-temperature polymorph, which does not stabilize as a ceramic at room temperature in single-component systems. The high-entropy tungstate (La0.2Nd0.2Gd0.2Tm0.2Y0.2)2WO6 was found to exhibit record oxygen-ion conductivity for this class of compounds (1.84 × 10-5 S/cm at 700 °C), which is an order of magnitude higher than that of α-Sm2WO6 and α-Dy2WO6, and more than twice as high as that of monoclinic Nd2WO6. In the catalytic reaction of oxidative methane condensation, the high-entropy composition provided the best balance of selectivity and activity (39%/12%) among ionic conductors, while monoclinic Eu2WO6, due to the ability of Eu to change its oxidation state, became the leader among electronic conductors (42%/14%), achieving the highest yield of target C2 products (5.9%).