Comparison for the crystal structure, synthesis and microwave dielectric properties of alkaline earth orthophosphates

A(3)(PO4)(2) compounds have different crystal structures when A sites are occupied by Ca or heavy alkaline earth metal atoms (Sr or Ba). The compounds with isomorphous crystal structure were synthesized by solid-state reaction method when the A-site atoms were Sr or Ba, and their crystal structure and microstructure of the sintered ceramics were investigated by X-ray powder diffraction (XRD) and scanning electron microscope (SEM), respectively. The microwave dielectric properties were measured using a network analyzer. It was found that Ba-3(PO4)(2) could be sintered at 1060 degrees C, while the alpha-Sr-3(PO4)(2) ceramics that has a smaller Sr2+ ionic radius, could be sintered at 1200 degrees C, and higher relative densities were obtained. The dielectric constant (epsilon) of the alpha-Sr-3(PO4)(2) is higher than that of Ba-3(PO4)(2), but Ba-3(PO4)(2) has a higher Q x f value than that of beta-Ca-3(PO4)(2) and alpha-Sr-3(PO4)(2), which could be interpreted by the differences in ionic polarizability and bond strength. The temperature coefficients of resonant frequency (tau(f)) for all samples with isomorphous crystal structure have positive values, ranging between +11 and +66 ppm degrees C-1. (C) 2012 Elsevier B.V. All rights reserved.