LAUSR

Abstract The electronic state and microwave dielectric properties of MgAl 2 O 4 prepared using solid-state (MA-S) and molten salt (MA-M) methods and those of Mg 0.4 Al 2.4 O 4 (M04A24) were investigated. The λ values, which correspond to the fraction of Al 3+ cations in tetrahedral sites, for MA-S, MA-M, and M04A24 were 0.23, 0.41, and 0.60, respectively. In molecular orbital calculations, a larger overlap was observed between Al-3 s or Al-3 p in tetrahedral sites and O-2 p orbitals for M04A24, and the bond order for Al O at tetrahedral sites of M04A24 (0.241) was higher than those for MA-S (0.178) and MA-M (0.205). The dielectric constant, e r , for M04A24 (7.6) was lower than those for MA-S and MA-M (both 7.9), and the highest quality factor, Q · f , was obtained for M04A24 (235, 800 GHz). It was found that the covalency of the Al O bonds in the M O 4 tetrahedra is closely related to the Q ･ f values of the present ceramics.