Enhanced temperature stability of tungsten bronze Ba4(La0.3Sm0.5Bi0.2)28/3Ti18O54 ceramics by (Al0.5Nb0.5)4+ ionic substitution

To enhance the temperature stability while preserving the high dielectric constant (ε_r) of Ba₄La_28/3Ti₁₈O₅₄ ceramics, a co-substitution strategy at the La/Ti site was implemented using the formula Ba₄(La_0.3Sm_0.5Bi_0.2)_28/3[Ti_1-x(Al_0.5Nb_0.5)_x]₁₈O₅₄ (x = 0~0.5). Crystallographic studies reveal that the introduction of small radius (Al_0.5Nb_0.5)⁴⁺ ion clusters forms a solid solution and causes a reduction in the crystal cell size. This leads to a blue-shift in the Raman peaks, particularly in the 200~400 cm^-1 range, which is associated with significant shifts due to the rotation and tilt of [TiO6] octahedra. This modification reduces the dielectric polarizability and results in the densification of the ceramic, contributing to a decrease in the ε_r value from 105.2 (x = 0) to 97.8 (x = 0.5). The reduction of Ti³⁺ ions and oxygen vacancies lowers the dielectric loss, and the growth and densification of grains are crucial for improving the quality factor (Q×f). The increased tilt angle of the [TiO6] octahedra primarily drives the reduction in the temperature coefficient of resonant frequency (τ_f). With the optimal inclusion of (Al_0.5Nb_0.5)⁴⁺ ions (x = 0.5), the temperature stability of Ba₄(La_0.3Sm_0.5Bi_0.2)_28/3Ti₁₈O₅₄ ceramics is significantly enhanced, maintaining excellent microwave dielectric properties with a ε_r value of 97.8, a high Q×f value of about 3670 GHz, and a τ_f of 10.5 ppm/^oC.