Synthesis and dielectric properties of tungsten-based complex perovskites

Ba₂MeWO₆ (Me=Mg, Ni, Zn) double perovskites were prepared by the conventional solid-state reaction in a wide temperature range. Single-phase ceramics were obtained only at low temperatures approximately 1200°C, whereas a small amount of second phases existed in the samples sintered at higher temperatures. All the compounds are characterized by the cubic perovskite structure (space group Fm3m) with a complete NaCl type ordering between B-site ions. Anomalous temperature variation of the dielectric loss tangent found in the Ba₂NiWO₆ perovskite is supposed to be connected with a dielectric relaxation due to electronic hopping within thermally activated Ni³⁺-6W^(6-1/6)+/W⁵⁺-6Ni^(2+1/6)+ clusters. Dielectric measurements showed that the other two perovskites—Ba₂ZnWO₆ and Ba₂MgWO₆—exhibit a positive value of the temperature coefficient of permittivity. Such temperature variation is assumed to be caused by a considerable influence of the second polar mode involving B-site ion vibrations on the low-frequency dielectric properties.