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

Abstract

Crystal structure and dielectric property of tungsten-bronze type microwave dielectric ceramics, i.e., BaOLa₂O₃4TiO₂ and Ba_6-3x (Nd, M)d_8+2x Ti₁₈O₅₄ (M = Y, Bi and x = 0.5, 0.7), are analysed. The optimum properties obtained in Ba(Nd_1-xBi_x)₂O₃4TiO₂ were ε_r = 89–92, Qf = 5855–6091 GHz, and τ_f = −7–+7 ppm/°C x = 0.04–0.08. The Y-substitution in BaO(Nd_1-xY_x)₂O₃4TiO₂ reduces the dielectric constant ε_r. Both the Y and Bi substitutions make τ_εr positive. The relative dielectric constant ε_r and temperature coefficient τ_εr are 109.5 and −180 ppm/°C in BaOLa₂O₃4TiO₂, 76 and +40 ppm/°C in BaO(Nd_0.77Y_0.23)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 Å). 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 τ_εr and octahedron tilting in tungsten-bronze type material is discussed in relation with complex perovskite structure.