Influence of (Mg1/3Nb2/3) complex substitutions on crystal structures and microwave dielectric properties of Li2TiO3 ceramics with extreme low loss

A systematic investigation of the Li 2 Ti 1-x (Mg 1/3 Nb 2/3 ) x O 3 (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 Li 2 Ti 1-x (Mg 1/3 Nb 2/3 ) x O 3 ceramics decreased linearly from 21.0 to 18.6, +27.1 to −19.4 ppm/°C, respectively. The Li 2 Ti 0.75 (Mg 1/3 Nb 2/3 ) 0.25 O 3 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/ o C. Moreover, the burying sintering process can reduce the volatilization of lithium so that the porosity of Li 2 Ti 0.75 (Mg 1/3 Nb 2/3 ) 0.25 O 3 ceramic was reduced effectively, which made Li 2 Ti 0.75 (Mg 1/3 Nb 2/3 ) 0.25 O 3 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.