Thermally-stimulated defect relaxations and microwave/terahertz dielectric response of La,Al co-doped (Ba,Sr)La4Ti4O15 ceramics

Abstract

The thermally-stimulated relaxation of defects in La,Al co-substituted (Ba,Sr)La4Ti4O15 dielectric ceramics with hexagonal perovskite structure were investigated systematically with the thermally stimulated depolarization current (TSDC) measurements. The ion substitution decreased the lattice parameters and the bond lengths of the cations and O2−. TSDC spectra indicated the main extrinsic defects in the ceramics were oxygen vacancies and the concentration of oxygen vacancies decreased with the increasing ion substitutions, which was derived from the decrease in the lattice parameters. The terahertz time-domain spectra indicated the damping factor of lattice vibration was reduced with the decline of the concentration of oxygen vacancies, which decreased the microwave/terahertz dielectric loss. High-performance (Ba0.2Sr0.8)0.75La4.25Ti3.75Al0.25O15 (εr = 43.9, Q × f = 84300 GHz, τf = -18.0 ppm/°C) ceramics were obtained. The correlations between crystal structures, defect behaviors and dielectric properties discussed in this work could provide guidance for the modification of microwave dielectric ceramics.