Effects of Lattice Evolution and Ordering on the Microwave Dielectric Properties of Tin-Modified Li<sub>3</sub>Mg<sub>2</sub>NbO<sub>6</sub>-Based Ceramics
Xing Zhang, Xiao Zhang, Zixuan Fang, Zhe Xiong, Hongyu Yang, Shuren Zhang, Bin Tang
Abstract
In this contribution, the evolution of the crystal structure and the consequent effects on the microwave dielectric responses of the Sn4+-substituted Li3Mg2–x/3SnxNb1–2x/3O6 (0 ≤ x ≤ 1.5) ceramics were investigated as a function of the doping content. Interestingly, a wide range of solid solution (0 ≤ x ≤ 1.1) was obtained by the complex substitutions of Sn ions, and a composition-driven phase transition from the orthorhombic to cubic phase accompanied by an order–disorder transformation occurring in the range of 0.1 ≤ x ≤ 0.7. During the process of the phase transition, high-resolution transmission electron microscopy images showed the presence of a coherent phase boundary between the orthorhombic and cubic phases, which was formed owing to their similar rock-salt crystal configurations and small mismatches in the subcell–lattice parameters. Besides, the electron diffraction patterns indicated that the specimen with relatively low Sn concentration (x = 0.2) contained reconstructed superlattices. The unique layered structure of the end-member Li3Mg2NbO6 constrained the form of the substitution for the dopant ions, where the nonequivalent ions (Sn4+) were expected to enter into the Nb–Mg–Nb clusters in the Nb-rich layers preferentially, which was likely to be the cause of the formation of the reconstructed superlattices. The substitution-induced superlattices were highly related to the low dielectric loss of the specimens, and a small amount of Sn doping (0 ≤ x ≤ 0.3 mol) also lowered the internal strain of the samples. As the Sn concentration increased from 0 to 0.3, the quality factors (Q × f) were significantly enhanced from 91,700 to 118,700 GHz with the emergence of the reconstructed superlattices and the lowered internal strain.