Dielectric properties and crystal structure of Ba6−3x(Nd, M)8+2xTi18O54 (M = La, Bi, Y) microwave ceramics

Crystal structure and dielectric property of tungsten-bronze type microwave dielectric ceramics, i.e., BaOLa2O34TiO2 and Ba6-3x (Nd, M)d8+2x Ti18O54 (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd1-xBix)2O34TiO2 were er = 89–92, Qf = 5855–6091 GHz, and τf = −7–+7 ppm/°C x = 0.04–0.08. The Y-substitution in BaO(Nd1-xYx)2O34TiO2 reduces the dielectric constant er. Both the Y and Bi substitutions make τer positive. The relative dielectric constant er and temperature coefficient τer are 109.5 and −180 ppm/°C in BaOLa2O34TiO2, 76 and +40 ppm/°C in BaO(Nd0.77Y0.23)2O34TiO2, respectively. The crystal structures were refined by Rietveld method using x-ray and neutron diffraction data. The most reliable results were obtained by refining the cation positions using the x-ray data and the oxygens from the neutron with a superlattice structure model Pnam(c-axis ≈ 7.6 A). The refined structures show that the a/c ratios are related to the apical oxygen displacements of the Ti–O octahedra. The substitution of the small radius atom, Y, produced a structure of severely tilted and distorted Ti–O octahedra and large a/c ratio, while the large radius atom, La, small a/c ratio. Differential scanning calorimetry analysis showed heat anomaly indicating suspected phase transition in these materials. The relation between τer and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure.