Relationship between crystal structure and dielectric properties of non-stoichiometric Mg 2 + x Al 4 + y Si 5 + z O 18 -based microwave ceramics

The effects of non-stoichiometry on the phase composition, crystal structure, and dielectric properties of Mg 2 + x Al 4 + y Si 5 + z O 18 ceramics were systematically studied. The stoichiometry deviation of Mg 2 + x Al 4 + y Si 5 + z O 18 ceramics leads to a change in the [MgO 6 ] oxygen octahedron and symmetry of crystal structure, which has a significant impact on their dielectric properties. The theoretical polarizability improves linearly with the increase of stoichiometric ratio deviation, resulting in a slight increase of permittivity from 4.93 to 5.08. The center symmetry of the [(Si 4 Al 2 )O 18 ] hexagonal ring in Mg 2 + x Al 4 + y Si 5 + z O 18 ceramics plays a crucial role in determining the Q × f value. The enhanced center symmetry of [(Si 4 Al 2 )O 18 ] hexagonal ring from asymmetric structure to centrosymmetric structure improves the Q × f values of Mg 2 + x Al 4 + y Si 5 + z O 18 ceramics. The reduction of τ f value is correlated to the decrease of Mg–O bond value and the increase of [MgO 6 ] oxygen octahedral distortion. The Mg 2.2 Al 4 Si 5 O 18 ceramic presents excellent dielectric properties at 1440 °C for 4h: ε r = 4.97, Q×f = 100,268 GHz, and τ f = −12.07ppm/ °C.