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., BaOLa(2)O(3)4TiO(2) and Ba6-3x(Nd, M)d(8+2x)Ti(18)O(54) (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd1-xBix)(2)O(3)4TiO(2) were epsilon(r) = 89-92, Qf = 5855-6091 GHz, and tau(f) = -7-+7 ppm/degrees C x = 0.04-0.08. The Y-substitution in BaO(Nd1-xYx)(2)O34TiO2 reduces the dielectric constant epsilon(r). Both the Y and Bi substitutions make tau(epsilon r) positive. The relative dielectric constant epsilon(r) and temperature coefficient tau(epsilon r) are 109.5 and -180 ppm/degrees C in BaOLa(2)O(3)4TiO(2), 76 and +40 ppm/degrees C in BaO(Nd0.77Y0.23)(2)O(3)4TiO(2), 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 approximate to 7.6 Angstrom). 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 tau(epsilon r) and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure. (C) 2000 Kluwer Academic Publishers.