Cation Valences and Multiferroic Properties of EuTiO₃ Co-Doped with Ba and Transition Metals of Co/Ni

Eu_1-BaTi_1-MO₃ (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray photoelectron spectroscopy revealed the mixed valences of Eu²⁺/Eu³⁺ and Ti⁴⁺/Ti³⁺ in EuTiO₃ and Eu_0.7Ba_0.3TiO₃, as well as some oxygen vacancies required to keep the charge neutrality. The co-doping of Co²⁺/Ni²⁺ in Eu_0.7Ba_0.3TiO₃ resulted in the disappearance of oxygen vacancies, as a result of a reduction in Ti³⁺ numbers and an increase in Eu³⁺ numbers. On the other hand, Ba²⁺ doping led to an increased lattice parameter due to its larger ionic size than Eu²⁺, whereas the Co²⁺/Ni²⁺ co-doping resulted in smaller lattice parameters because of the combined effects of ionic size and variation in the oxygen-vacancy numbers. Eu_0.7Ba_0.3TiO₃ exhibited a clear ferroelectricity, which persisted in the Co²⁺/Ni²⁺ co-doped samples until the doping levels of y = 0.05 and 0.10, respectively. Eu_0.7Ba_0.3TiO₃ remained to be antiferromagnetic with a reduced transition temperature of 3.1 K, but co-doping of Co²⁺/Ni²⁺ turned the samples from antiferromagnetic to ferromagnetic with transition temperatures of 2.98 K and 2.72 K, respectively. The cause for such a transition could not be explained by the larger lattice volume, oxygen vacancies and mixed valences of Eu²⁺/Eu³⁺, which were proposed in previous works. Instead, it was more likely to arise from a large asymmetric distortion of the Eu-O polyhedron introduced by the aliovalent doping, which promotes the admixture of Eu 5d and 4f states.