Crystal structure, Raman vibrational characteristics, and microwave dielectric properties of Mg_2−xCo_xTiO₄ ceramics

A series of Mg_2−xCo_xTiO₄ (x = 0.0, 0.5, 1.0, 1.5, 2.0) ceramics were synthesized by a conventional solid-state reaction route. The crystal structure, Raman vibrational characteristics, sintering behavior, microstructure, and microwave dielectric properties were systematically investigated. X-ray diffraction patterns and Raman spectroscopy indicated that the Mg_2−xCo_xTiO₄ ceramics all exhibited a single-phase structure with cubic spinel (space group: Fd \(\overline{3 }\) $\overline{3}$ m). The lattice parameters gradually increased with the continuous doping of Co²⁺ ions according to the Rietveld refinement results. Structure–property relationships were established based on Clausius–Mossotti equations, Raman spectroscopy, and bond valence theory. The ε_r values of Mg_2−xCo_xTiO₄ ceramics were significantly affected by the ionic polarizability and the [Mg₁/Co₁O₄] stretching vibration. The variation tendency of the Q × f values was closely related to the densification, and packing fraction. Moreover, the “rattling” effect at the A-site, the bond valence of the B-site, and the oxygen bond valence played an important role in controlling the τ_f values of Mg_2−xCo_xTiO₄ single-phase ceramics. An excellent microwave dielectric property of ε_r = 15.88, Q × f = 38,157 GHz, and τ_f = − 32.21 ppm/°C can be obtained for Mg_2−xCo_xTiO₄ ceramics with x = 1.0 sintered at 1350 °C for 4 h.