Structural evolution and microwave dielectric properties of a novel Li3Mg2−x/3Nb1−2x/3TixO6 system with a rock salt structure

Abstract

In this study, the Li₃Mg_2−x/3Nb_1−2x/3Ti_xO₆ (0 ≤ x ≤ 0.6) solid solution was designed and synthesized by the solid-state reaction method. The ordering degree decreased with the increasing Ti content, and the ordered orthorhombic superstructure was continuously transformed to the disordered MgO-like cubic subcell structure with no other impure phase observed in the range of 0.12 ≤ x ≤ 0.38. During the phase transition process, the ordered domains were found in orthorhombic superstructures by TEM images. According to the variation of Raman patterns, the B_1g, B_2g and B_3g vibration modes which derive from Nb–Nb repulsive force in 8a sites were assigned to the band at 480 cm⁻¹. The relative densities and dielectric constants showed a similar nonlinear variation with the increase of Ti content. However, as x increased from 0 to 0.4, the phase transition contributed to the huge improvement of Q × f values from 82 338 GHz to 125 357 GHz (about 50%), but it also led to a slight decrease in the temperature coefficient of resonant frequency (τ_f). After being sintered at 1260 °C for 6 h, the Li₃Mg_1.87Nb_0.73Ti_0.40O₆ (x = 0.4) ceramics showed excellent microwave properties: ε_r = 15.09, Q × f = 125 357 GHz and τ_f = −31.7 ppm °C⁻¹.