Bond ionicity, lattice energy, bond energy and the microwave dielectric properties of non-stoichiometric MgZrNb2+xO8+2.5x ceramics

The intrinsic relationship between crystal structure, surface topography, P-V-L theory and dielectric properties of non-stoichiometric MgZrNb2+xO8+2.5x (−0.08 ≤ x ≤ 0.08) ceramics prepared by a normative solid-state reaction route were determined. X-ray diffraction patterns demonstrated that the appropriate addition of excess or insufficient niobium ions could not destroy the crystal phase, but change the lattice parameters and unit cell volume. Meanwhile, the formation of pure phase with monoclinic wolframite structure and P2/c space group was confirmed. The Rietveld refinement was used to obtain structure parameters and investigate the crystal structure of the specimens. SEM showed that insufficient niobium ions didn't show any impact on the dense structure, and the excess niobium ions promoted the growth of grains, but caused the appearance of undesired rough surfaces. The microwave properties as a function of the niobium ions content were found to be closely related to bond ionicity, lattice energy and bond energy calculated according to the P-V-L theory. As a result, when x was taken as −0.04 for the MgZrNb2+xO8+2.5x ceramics sintered at 1320 °C, the best dielectric properties, εr = 25.62, Q × f = 80828.1 GHz, τf = −44.64 ppm/°C, were successfully obtained.