Oxygen vacancy regulation and its high frequency response mechanism in microwave ceramics

Abstract

Li₂ZnTi_3−xM_xO₈ {M = Al³⁺, Nb⁵⁺, (Al_0.5Nb_0.5)⁴⁺, (Zn_1/3Nb_2/3)⁴⁺, and (Li_1/4Nb_3/4)⁴⁺} systems were fabricated by a facile solid-state reaction method. According to the substitution rule of radius and valence, all of the doping ions {Al³⁺, Nb⁵⁺, (Al_0.5Nb_0.5)⁴⁺, (Zn_1/3Nb_2/3)⁴⁺, and (Li_1/4Nb_3/4)⁴⁺} should occupy Ti⁴⁺ sites. EPR spectra showed that paramagnetic Ti³⁺ centers and oxygen vacancies co-existed in the pure Li₂ZnTi₃O₈ ceramics. The oxygen vacancies in Li₂ZnTi₃O₈ were attributed to the deoxidation process during high temperature sintering. The inhibitory effects of acceptor ion, donor ion and co-doped ions on oxygen vacancies were related to the lattice energy (U) of O(1). An appropriate amount of (Al_0.5Nb_0.5)⁴⁺, (Zn_1/3Nb_2/3)⁴⁺ and (Li_1/4Nb_3/4)⁴⁺ greatly reduced the microwave dielectric loss of the Li₂ZnTi₃O₈ systems. And, the microwave dielectric loss deteriorated seriously in Al³⁺-doped systems. For the first time, a systematic exposition on the response process of dielectric loss in Li₂ZnTi₃O₈ ceramics was discussed based on the ac impedance analysis and terahertz time-domain spectroscopy (THz-TDS) analysis in detail. Deeper analysis revealed that the absorption of structural phonon oscillation and long-range movement of weakly bound defects () were two major contributors to the microwave dielectric loss.