Crystal structure, chemical bond, and microwave dielectric properties of low-εr LiAlO2 ceramic doped with Be2+ and Ga3+

The solid-phase method was utilized to synthesize LiAl 1- x Be x O 2- x /2 and LiAl 1- x Ga x O 2 ceramics. By Phillips-Van Vechten-Levine (P-V-L) theory, a strong correlation between chemical bond characteristics and microwave dielectric properties was established. X-ray diffraction (XRD) analysis confirmed that all components formed single-phase solid solution with tetragonal structure. Using Be 2+ and Ga 3+ as substitutes, the differences in ionic radii produced diametrically opposite changes in cell volume. The relative density increased to 97.61 % (98.57 %) with the addition of Be 2+ (Ga 3+ ). According to the P-V-L theory, the variation of ε r was influenced by bond ionization. The introduction of different ions caused bond lengths variations, which in turn affected the lattice energy fluctuations. Importantly, this change more than doubled Q×f from 16,268 GHz to 37,015 GHz. Furthermore, the evolution of τ f can be elucidated by the bond energy. Sintered at 1250 °C, both LiAl 0.96 Be 0.04 O 1.98 and LiAl 0.98 Ga 0.02 O 2 demonstrated outstanding microwave dielectric properties, characterized by ε r  = 5.83–6.08, Q×f  = 32,727∼37,015 GHz, and τ f  = −134.8∼-112.4 ppm/°C.