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Low-loss microwave dielectric ceramics are of great significance for fifth-generation telecommunication (5G) devices used for higher frequencies, while reducing the dielectric loss in the microwave band is still a technical challenge. Lattice dynamics and terahertz response are closely related to the polarization of dielectrics, and can be used as effective methods to explore the loss mechanism of microwave dielectrics. Here, Al3+ was doped into the conventional high-permittivity (ε r) Ca 1- x Sm 2 x /3 TiO 3 (x = 0.4) ceramic system, enhancing the Q × f values significantly. Raman spectra, thermally stimulated depolarization currents (TSDC) and terahertz time-domain spectra (THz-TDS) together showed that the stiffness of cation vibration became stronger when z value was larger, indicating a smaller damping of the lattice vibration, and leading to lower extrinsic losses and higher Q × f values. The relationship between lattice dynamics, defect behavior and microwave dielectric loss will provide a significant reference for the development of low-loss microwave dielectric ceramics. • Medium/high- ε r dielectrics of microwave/terahertz devices are developed. • The Q × f values in the microwave and terahertz bands are essentially constant. • Bond strength, defects, lattice vibration and dielectric loss have strong correlation. [ABSTRACT FROM AUTHOR]

• The defect dipoles and oxygen vacancies were supposed to be the main defects in La,Al co-doped (Ba,Sr)La 4 Ti 4 O 15 ceramics. • The oxygen vacancies in the ceramics were affected by the lattice parameters. • The oxygen vacancies in the ceramics dominated the dielectric loss in microwave/terahertz frequency ranges. The thermally-stimulated relaxation of defects in La,Al co-substituted (Ba,Sr)La 4 Ti 4 O 15 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 (Ba 0.2 Sr 0.8) 0.75 La 4.25 Ti 3.75 Al 0.25 O 15 (ε r = 43.9, Q × f = 84300 GHz, τ f = -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. [ABSTRACT FROM AUTHOR]