Microwave dielectric properties and cation distribution of spinel-structured Mg0.4Al2.4− x Ga x O4 ceramics with cation defect

The spinel-structured Mg0.4Al2.4−xGaxO4 ceramics were obtained in the composition range of 0.4–1.2. The coexistence of spinels with and P4332 was suggested from the X-ray powder diffraction and crystal structure refinement. The cation distribution in the spinels was characterized by 27Al and 71Ga solid-state NMR and it was found that Al3+ cations preferentially occupy the octahedral site, while the tetrahedral site is occupied by Ga3+ cations, resulting in the increase in the fraction of Ga3+ cations in the tetrahedral site from 0.30 to 0.69. No remarkable variation in εr by the Ga substitution for Al (εr ≒ 8.1) was observed. In spite of the coexistence of the spinels with Fd3m and P4332, the Q · f value gradually increased from 163,437 to 191,340 GHz in the composition range of 0.4–1.2; the increase in the degree of inversion may be related to the increase in Q · f.