Synthesis, structure, and characterization of new low-firing microwave dielectric ceramics: (Ca1−3xBi2xΦx)MoO4

Abstract

A series of A-site deficient scheelite ceramics (Ca_1−3xBi_2xΦ_x)MoO₄ (x = 0.005, 0.015, 0.025, 0.035, 0.05, 0.1, 0.15, and 0.2, and Φ: A-site vacancy) were synthesized via the solid state reaction route. The structures were analyzed using a combination of X-ray diffraction and X-ray absorption fine structure spectroscopy (Mo K-edge and Bi L₃-edge) to determine average and local structures. A series of defective scheelite (Ca_1−3xBi_2xΦ_x)MoO₄ compositions can be formed as a solid solution, and local structures of Mo and Bi indicate that a MoO₄ tetrahedron and a BiO₈ polyhedron become more distorted with the x value. The large change in the Bi–O1 (the first shell) and Bi–O2 (the second shell) distances is an important insight into the nature of the defective structures. The statistical disorder of a Bi–O bond is one order of magnitude larger than that of a Mo–O bond. The microstructures and microwave dielectric properties were investigated by scanning electron microscopy and through network analyzer resonance studies. All the compositions can be sintered well below 900 °C. With slight Bi substitutions (x = 0.005 and 0.015), the samples exhibit improved Q × f values. At x = 0.15, temperature stable (TCF = −1.2 ppm per °C) low-firing (ST = 700 °C) microwave dielectric materials were obtained with a permittivity of 21.2 and a Q × f value of 29 300 GHz. The factors affecting dielectric properties are associated with the local structures of Mo and Bi across the solid solution.