Symmetry of hexagonal ring and microwave dielectric properties of (Mg1−xLnx)2Al4Si5O18+x (Ln = La, Sm) cordierite-type ceramics

Cordierite-type (Mg1−xLnx)2Al4Si5O18+x (Ln = La, Sm, 0 ≤ x ≤0.35) ceramics with low permittivity (εr) were fabricated by traditional solid state reaction method. Analysis of X-ray diffraction and Raman spectra demonstrate that the symmetry of hexagonal rings strongly affects the microwave dielectric properties of (Mg1−xLnx)2Al4Si5O18+x cordierite ceramics. [(Mg1−xLnx)6O36] external hexagonal rings in cordierite ceramics tend to be distorted as Ln3+ ions substitutes Mg2+ ions, and the shapes of [(Si4Al2)O18] inner hexagonal rings are alternatively tuned from elliptical to circular. Compared with pure Mg2Al4Si5O18 cordierite ceramics, the improvement of [(Si4Al2)O18] ring centro-symmetry and the uniform distribution of grain sizes obviously enhance the Qf value of (Mg1−xLnx)2Al4Si5O18+x cordierite type ceramics. The optimum microwave dielectric properties with εr = 6.7, Qf = 78,500 (@14.3 GHz), τf = −22 ppm/°C were obtained by (Mg0.95La0.05)2Al4Si5O18+0.05 ceramics. Relative thermal stability with εr = 6.8, Qf = 24,800 GHz (@13.2 GHz), τf = −17 ppm/°C were obtained by (Mg0.8Sm0.2)2Al4Si5O18+0.2 ceramics.