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A systematic study on the site occupancy of rare-earth cations in Ba-rich and Ti-rich BaTiO3 fired in reducing atmosphere is performed. The corresponding lattice relaxation is used as an indicator of site occupancy and, to a lesser extent, defect structures. Accurate lattice parameters are obtained from X-ray diffraction data that is analyzed with the maximum likelihood method to account for correlated errors in the c and a lattice parameters. Comparisons of lattice volume as a function of ionic radius of the dopant reveals three regimes, with the intermediate sized cations (0.087 nm \leqslantr \leqslant0.094 nm where r is the ionic radius in six-fold coordination) demonstrating amphoteric behavior (occupying A- or B-sites). Defect chemistry analysis of site occupancy links the importance of metal vacancy ratios and oxygen vacancy concentrations with site occupancy. The trends predicted from the defect chemistry analysis are consistent with the observed lattice relaxations.

Air-fired barium titanate samples doped with cerium, neodymium, samarium, gadolinium, dysprosium, erbium, or ytterbium were examined by electron paramagnetic resonance (EPR). Reducing atmosphere-fired europium-doped barium titanate was also investigated with EPR. Each dopant was studied in both Ba- and Ti-rich (Ba/Ti = 1.01, 0.99) samples. Point charge calculations were used to predict the EPR spectrum of each lanthanide in A- and B-sites. Different EPR spectra are expected for A- versus B-site substitution when Ce3+, Sm3+, Dy3+, and Yb3+ are the dopants. The experimentally observed Ba/Ti doping behavior of Ce3+ in BaTiO3 suggests that as a 3+ cation it is on the A-site. No EPR active signal was observed for Sm3+ in BaTiO3. Eu2+ and Gd3+, as previously discussed in the literature, were found to be an A-site dopant and amphoteric, respectively. Dy3+ was found to be a B-site dopant with an EPR signal intensity suggesting amphoteric behavior, whereas Yb3+ showed only B-site occupancy. Nd3+ and Er3+ could not easily be assigned to a particular site by EPR methods alone. We also discuss the lanthanide dopant's effect on the observed levels of titanium vacancies, barium vacancies, and Mn2+ impurities.