Preferential site occupancy, octahedral stabilization energy, and microwave dielectric properties of mixed spinel Mg0.95X0.05Al2O4

In this paper, different divalent ions (X = Ni, Cu, Mg, Co, Zn) are doped to replace Mg2+ at the tetrahedral sites in mixed spinel Mg0.95X0.05Al2O4. These substitutions induce variations in the degree of inversion/preferential occupation at the A/B lattice site in MgAl2O4 ceramics because the octahedral stabilization energies change as a function of dopant. According to crystal field theory, the number of d-orbital electrons has a significant impact on the stability of octahedral sites. Therefore, OSPE values vary between 0 Dq and 0.845 Dq for different elements. The higher the OSPE value, the greater the degree of inversion, and the higher the quality factor (Q × f). The dielectric constant (εr) and temperature coefficient of resonant frequency (τf) of Mg0.95X0.05Al2O4 ceramics changes as a function of the ionic polarizability and B-site bond valence. Mg0.95Ni0.05Al2O4 with a higher OSPE (8.45 Dq) and degree of inversion of A/B sites (λ = 0.221), has the better set of microwave dielectric properties (εr = 8.39, Q × f = 78,338 GHz, τf = −66.8 ppm/°C). The adjustment of stability energy of octahedral sites is favorable to improve the microwave dielectric properties of MgAl2O4 spinel.