Self-adjustable site occupations between Ba-site Tb3 + and Ti-site Tb4 + ions in terbium-doped barium titanate ceramics

• A broad g = ~ 6.5 EPR signal of Ti-site Tb4 + is first discovered in Tb-doped BaTiO3.
• Raman charge effect at 805–833 cm− 1 and g = 2.004 EPR signal relates to Ba-site Tb3 +.
• Tb ions are self-adjustable in site occupations due to different Ba/Ti ratios.
• Defect chemistry of Tb-doped BaTiO3 ceramics is discussed.
• 5% Tb-doped BaTiO3 with Ba/Ti = 0.987 approaches to X5R dielectric specification.

Incorporation of Tb ions into the BaTiO3 lattice was studied on ceramic samples with 5% Tb and Ba/Ti ratios of 0.987–1.053, using electron paramagnetic resonance (EPR) in combination with X-ray diffraction (XRD), Raman spectroscopy (RS), scanning electron microscopy (SEM), and dielectric measurements. The EPR results provided direct evidence of the existence of Tb ions at Ti sites as Tb4 +, which was characterized by a broad signal with g = ~ 6.5 and an increased concentration of Ti-site Tb4 + ions and a corresponding increase in the Ba/Ti ratio. The Raman charge effect at 805–833 cm− 1 and an EPR signal at g = 2.004 associated with ionized Ti-vacancy defects provided the evidence for the existence of Tb ions at Ba sites as Tb3 +. Because of the change in the Ba/Ti ratio, Tb ions in BaTiO3 exhibited a self-adjustable amphoteric behavior with the mixed valence states of Ba-site Tb3 + and Ti-site Tb4 + utilized to preserve lattice electroneutrality. The ceramic density, morphology of grains, dielectric properties, and Raman charge effect were sensitive to the Ba/Ti ratio. The sample with a Ba/Ti ratio of 0.987 has evident advantages: a higher density, smooth-surfaced grains, lower dielectric loss, and low-temperature dielectric stability approaching to an X5R specification. Defect chemistry of Tb-doped BaTiO3 is discussed.