Structure, defects, and microwave dielectric properties of Al-doped and Al/Nd co-doped Ba₄Nd_9.33Ti₁₈O₅₄ ceramics

Low-loss tungsten-bronze microwave dielectric ceramics are dielectric materials with potential application value for miniaturized dielectric filters and antennas in the fifth-generation (5G) communication technology. In this work, a novel Al/Nd co-doping method of Ba₄Nd_9.33Ti₁₈O₅₄ (BNT) ceramics with a chemical formula of Ba₄Nd_9.33+z/3Ti_18-zAl_zO₅₄ (BNT-AN, 0 ≤ z ≤ 2) was proposed to improve the dielectric properties through structural and defect modulation. Together with Al-doped ceramics (Ba₄Nd_9.33Ti_18-zAl_4z/3O₅₄, BNT-A, 0 ≤ z ≤ 2) for comparison, the ceramics were prepared by a solid state method. It is found that Al/Nd co-doping method has a significant effect on improving the dielectric properties compared with Al doping. As the doping amount z increased, the relative dielectric constant (ε_r) and the temperature coefficient of resonant frequency (τ_f) of the ceramics decreased, and the Q×f values of the ceramics obviously increased when z ≤ 1.5. Excellent microwave dielectric properties of ε_r = 72.2, Q×f = 16,480 GHz, and τ_f = +14.3 ppm/℃ were achieved in BNT-AN ceramics with z = 1.25. Raman spectroscopy and thermally stimulated depolarization current (TSDC) technique were firstly combined to analyze the structures and defects in microwave dielectric ceramics. It is shown that the improvement on Q×f values was originated from the decrease in the strength of the A-site cation vibration and the concentration of oxygen vacancies ( ${\text{V}}_{\text{O}}^{\cdot \cdot }$), demonstrating the effect and mechanism underlying for structural and defect modulation on the performance improvement of microwave dielectric ceramics.