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 Mg 2+ at the tetrahedral sites in mixed spinel Mg 0.95 X 0.05 Al 2 O 4 . These substitutions induce variations in the degree of inversion/preferential occupation at the A/B lattice site in MgAl 2 O 4 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 ( ε r ) and temperature coefficient of resonant frequency ( τ f ) of Mg 0.95 X 0.05 Al 2 O 4 ceramics changes as a function of the ionic polarizability and B -site bond valence. Mg 0.95 Ni 0.05 Al 2 O 4 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 ( ε r  = 8.39, Q  ×  f  = 78,338 GHz, τ f  = −66.8 ppm/°C). The adjustment of stability energy of octahedral sites is favorable to improve the microwave dielectric properties of MgAl 2 O 4 spinel.