Influence of (Mg_1/3Nb_2/3) complex substitutions on crystal structures and microwave dielectric properties of Li₂TiO₃ ceramics with extreme low loss

A systematic investigation of the Li₂Ti_1-x(Mg_1/3Nb_2/3)_xO₃ (0.1 ≤ x ≤ 0.35) solid solutions synthesized by traditional solid-state reaction method is reported in this work. In the composition range of 0.1 ≤ x ≤ 0.25, a monoclinic rock salt structured solid solution was formed. When x increased to 0.3, a phase transition from monoclinic to cubic phase, along with an order-disorder phase transition, was observed. With x increased from 0.1 to 0.35, the microwave permittivity (ε_r) and temperature coefficient of resonant frequency (TCF) of the Li₂Ti_1-x(Mg_1/3Nb_2/3)_xO₃ ceramics decreased linearly from 21.0 to 18.6, +27.1 to −19.4 ppm/°C, respectively. The Li₂Ti_0.75(Mg_1/3Nb_2/3)_0.25O₃ ceramic sintered at 1170 °C shows high performance of microwave dielectric properties with a ε_r ∼19.6, a Qf (Q = quality factor = 1/dielectric loss; f = resonant frequency) ∼109,770 GHz (at 7.7 GHz) and a near zero TCF ∼ + 1.2 ppm/^oC. Moreover, the burying sintering process can reduce the volatilization of lithium so that the porosity of Li₂Ti_0.75(Mg_1/3Nb_2/3)_0.25O₃ ceramic was reduced effectively, which made Li₂Ti_0.75(Mg_1/3Nb_2/3)_0.25O₃ ceramic promising for future applications. Selected area electron diffraction patterns, high-resolution transmission electron microscopy, Raman and far-infrared spectra were employed to study the relation between crystal structure and microwave dielectric properties in detail.