Effects of LiF on crystal structure, cation distributions and microwave dielectric properties of MgAl2O4

In this work, MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics were synthesized by solid-state reaction method. The phase information of a spinel structure was delineated by XRD. The crystal structure of MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics were discussed with the help of Rietveld refinements. As x increased from 0 to 6.4 mol. %, the tetrahedral sites in MgAl2O4 ceramics gradually were occupied by Al3+ cations, which is confirmed by 27Al NMR and Raman spectra. Such the occupation of Al3+ cations increased the covalency (fc/s) value of M-O (M = Mg, Al) bond in MO4 tetrahedral of MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics. LiF is mainly used as a sintering aid to improve the formation of the MgAl2O4 ceramics. In order to obtain high Q×f value spinel ceramics, LiF must be removed before the final stage of densification. In addition, the dielectric constant (εr), grain size and quality factor (Q×f) value are significantly affected by the LiF content. However, the temperature coefficient of resonance frequency (TCf) values were independent of LiF content. MgAl2O4 + xLiF (x = 0, 3.2, 6.4 mol. %) ceramics with x = 6.4 has excellent microwave dielectric properties when sintered at 1575 °C for 8 h: εr = 8.36, Q×f = 99,900 GHz, TCf = − 61.57 ppm/°C.