Lattice Evolution, Ordering Transformation and Microwave Dielectric Properties of Rock-Salt Li3 +xMg 2-2xNb 1-XTi 2xO6 Solid-Solution System: A Newly Developed Pseudo Ternary Phase Diagram

Abstract

The new types of multi-component Li3+xMg2-2xNb1-xTi2xO6 (0≤x≤1) solid-solution ceramics were designed based on the Li2TiO3-Li3NbO4-MgO pseudo ternary phase diagram and studied for millimeter-wave applications. The ceramics showed a full range of solid solubility between the Li3Mg2NbO6 and Li2TiO3 endmembers. As the doping content (x) increased, we detected the phase transitions among the orthorhombic, cubic, and monoclinic phase driven by the compositional changes, as well as accompanied with an order-disorder-order transformation. Besides, the smooth orthorhombic-cubic phase transition interfaces were observed in the electron diffraction images for the sample with the low dopant concentration (x=0.2), and the reconstructed superlattices based on the orthorhombic or the cubic structure were both observed nearby the orthorhombic-cubic phase transition interface. The change of Q×f values showed a considerable increase with the emergence of the superlattices, and the enhancement of the Q×f values was related to the very small lattice vibrational anharmonicity in the structures that contain the reconstructed superlattices. Typically, when sintered at 1250 ℃ for 4 h, the ultra-low dielectric loss was achieved in the x=0.2 sample with the properties of er=16.1, Q×f=128,600 GHz (f=9.2 GHz) and τf=-30.4 ppm/℃, and the temperature stable properties were obtained in the x=0.9 sample with er=20.4, Q×f=90,300 GHz (f=7.9 GHz), and τf=2.9 ppm/℃.