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

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= similar to 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. (C) 2015 Elsevier B.V. All rights reserved.