Structure and microwave dielectric properties of Ba3Nb4-4xTi4+5xO21 ceramics with medium-high permittivity

Abstract

A series of Ba3Nb4-4xTi4+5xO21 (x = 0.1, 0.2, and 0.3) ceramics with hexagonal structure was synthesized by a solid-state reaction method. The relationships between the composition, crystal structure, and microwave dielectric properties were investigated in terms of ionic polarizability, packing factor, bond valence, octahedral distortion, transmission electron microscopy, Raman spectroscopy and infrared reflectivity spectra. As the x value increases from 0.1 to 0.2, the compounds crystallize in a hexagonal-bronze type (S.G. P63/mcm) structure, whereas the further increase of the x results in Ba6Nb2Ti14O39 appearing as a secondary phase for x = 0.3. The relative density and Q × f firstly increase to a maximum value and then decline, however, the εr and τf values slightly decrease as the x increases. It might be inferred that the excess cation from B-site jump into the C-site strengthening the lattice structure and reducing the dielectric loss and ionic polarizability. The microwave dielectric properties of Ba3Nb3.2Ti5O21 ceramic were obtained after sintering at 1210 °C for 6 h, with permittivity (εr) ∼60.3, quality factor (Q × f) = 9,400 GHz, temperature coefficient of resonant frequency (τf) ∼ 109.7 ppm/°C (10−6/°C). The intrinsic dielectric parameters of Ba3Nb3.2Ti5O21 with εr∼55, Q × f = 19,480 GHz were also obtained by infrared reflectivity spectra.