Bond characteristics and microwave dielectric properties of (Li_0.5Ga_0.5)²⁺ doped Mg₂Al₄Si₅O₁₈ ceramics

Cordierite ceramic is one kind of important electronic materials for radio frequency circuit. In this work, Mg_2-x (Li_0.5Ga_0.5)_xAl₄Si₅O₁₈ (0 ≤ x ≤ 0.1) ceramics with pure cordierite phase, were successfully synthesized via the traditional solid state reaction method. The dependence of microwave dielectric properties on the crystal structure was analyzed in detail by the complex chemical bond theory (P–V-L theory) and classical dielectric polarizability theory. The dielectric constant (ε_r) was mainly related to the density, bond ionicity and ionic polarizability of Mg_2-x (Li_0.5Ga_0.5)_xAl₄Si₅O₁₈ ceramics. The quality factor (Q × f) was in relationship to the lattice energy, atomic packing fraction and centrosymmetry of [Si₄Al₂O₁₈] hexagonal ring in cordierite crystal structure. The temperature coefficient of resonant frequency (τ_f) value was dominated by the bond energy and distortions of [MgO₆] octahedron. The maximum values of Q × f = 50,560 GHz and ε_r = 4.72 were obtained for undoped Mg₂Al₄Si₅O₁₈ and Mg_1.98Li_0.01Ga_0.01Al₄Si₅O₁₈, respectively.