Crystal chemistry and microwave dielectric properties of Ba3MNb2-xSbxO9 (M = Mg, Ni, Zn)

The effect of Sb5+ substitution on the crystal chemistry and dielectric properties of Ba3MNb2-xSbxO9 (M = Mg, Zn, Ni) was investigated using a combination of X-ray and neutron powder diffraction, and dielectric property measurements at microwave frequencies. Rietveld refinements were carried out to characterize the crystal structures of Ba3ZnSb2O9 and Ba3MgSb2O9, which have a 6H-BaTiO3-type structure. Analyses of X-ray diffraction data of intermediate compositions evolve from a 2:1 ordered perovskite solid solution, to a two-phase ordered perovskite and 6H region, and also a 6H solid-solution for increasing Sb-content. Analysis of the bond distances indicates that the significant strain present in the 2:1 ordered perovskites is relieved in the 6H structure. Differences in the bonding preferences between the Nb5+ (do) and Sb5+ (d(10)), combined with small changes in the ionic radii, influence the observed crystal chemistry. The magnitude and temperature dependence of the dielectric constant were lowest for compounds in the perovskite solid solution range with intermediate Sb5+ contents. Ba3MgNb1.75Sb0.25O9 (epsilon > 25, tau(f) approximate to 6 ppm K-1, and Qf > 96 000 GHz) and Ba3ZnNb1.625Sb0.375O9 (epsilon > 33, tau(f) approximate to-3 ppm K-1, and Qf > 44 900 GHz) display useful dielectric properties, rendering these materials suitable as lower-cost and lower-processing-temperature alternatives to perovskite tantalate ceramics.