Structural dependence of microwave dielectric performance of wolframite structured Mg1-xCaxZrNb2O8 ceramics: Crystal structure, microstructure evolution, Raman analysis and chemical bond theory

Wolframite-structured Mg 1-x Ca x ZrNb 2 O 8 (0−0.1) ceramics were synthesized through a solid-state procedure. Crystal refinement indicates a pure Mg 1-x Ca x ZrNb 2 O 8 ceramic with a wolframite structure. Ca 2+ substitution led to the redshift of the Ag mode at approximately 896 cm −1 . Additionally, Ca 2+ substitution could promote grain growth and contribute to microstructure evolution from a polyhedral shape to a rod shape. According to chemical bond theory, an appropriate Ca 2+ concentration can increase Nb O bond iconicity and Nb O bond lattice energy, which contributed to the great improvement in the dielectric constant and Q × f value. Additionally, the τ f value was affected by the bond valence and thermal expansion coefficient of the Mg O bond. The Mg 1-x Ca x ZrNb 2 O 8 ( x = 0.04) ceramics exhibited great improvement in the Q × f value: ε r = 25.21, Q × f = 116,000 GHz (@7.17 GHz) and τ f = −24.4 ppm/°C, which provides enormous potential for future millimeter-wave applications.