Relationship between crystal structure and dielectric properties of non-stoichiometric Mg2+xAl4+ySi5+zO18-based microwave ceramics

The effects of non-stoichiometry on the phase composition, crystal structure, and dielectric properties of Mg2 + xAl4 + ySi5 + zO18 ceramics were systematically studied. The stoichiometry deviation of Mg2 + xAl4 + ySi5 + zO18 ceramics leads to a change in the [MgO6] 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 [(Si4Al2)O-18] hexagonal ring in Mg2 + xAl4 + ySi5 + zO18 ceramics plays a crucial role in determining the Qxf value. The enhanced center symmetry of [(Si4Al2)O-18] hexagonal ring from asymmetric structure to centrosymmetric structure improves the Qxf values of Mg2 + xAl4 + ySi5 + zO18 ceramics. The reduction of tau(f) value is correlated to the decrease of Mg-O bond value and the increase of [MgO6] oxygen octahedral distortion. The Mg2.2Al4Si5O18 ceramic presents excellent dielectric properties at 1440 degrees C for 4h: epsilon(r) = 4.97, Qxf = 100,268 GHz, and tau(f) = -12.07ppm/ degrees C.