The relationship between the defect structure and dielectric properties of (La, Li) co-doped SrTiO₃

This work involved the structure and dielectric properties study of (La, Li) co-doped Sr_1−x−yLa_xLi_yTiO₃ (0 ≤ x ≤ 0.04, 0 ≤ y ≤ 0.04) ceramics prepared using the traditional solid-phase method. All the samples were compact ceramics with pure SrTiO₃ phase. The Rietveld refinement combined theoretical structure optimization indicated that the La preferred to occupy the Sr sites while the Li could substitute for both the Sr and Ti sites. With increasing the co-doping level, the lattice parameters initially increase and then decrease, while the concentration of oxygen vacancies in the co-doped sample increases. The Sr_0.92La_0.04Li_0.04TiO₃ (SLLT-8) exhibits a dielectric constant of 1530 with a loss of 0.04 (30 °C, @1 kHz) and good temperature stability ranging from 30 to 400 °C (Δ ε_r/ε₃₀ <  ± 25%, 100 kHz), as well as good frequency stability between 10³ and 10⁵ Hz (Δ ε_r/ε_1kHz <  ± 10%, 30 °C). The mechanism of the dielectric relaxation was analyzed through XPS, impedance spectra, and density functional theory (DFT) calculation. The results show that high dielectric constant and low loss are related to the electron pinning defect dipole (EPDD) effect induced by oxygen vacancy-related defect clusters in the lattice.