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

EuBaTiMO (M = Co or Ni) was sintered at 1400 °C under a reduction atmosphere. X-ray photoelectron spectroscopy revealed the mixed valences of Eu/Eu and Ti/Ti in EuTiO and EuBaTiO, as well as some oxygen vacancies required to keep the charge neutrality. The co-doping of Co/Ni in EuBaTiO resulted in the disappearance of oxygen vacancies, as a result of a reduction in Ti numbers and an increase in Eu numbers. On the other hand, Ba doping led to an increased lattice parameter due to its larger ionic size than Eu, whereas the Co/Ni co-doping resulted in smaller lattice parameters because of the combined effects of ionic size and variation in the oxygen-vacancy numbers. EuBaTiO exhibited a clear ferroelectricity, which persisted in the Co/Ni co-doped samples until the doping levels of = 0.05 and 0.10, respectively. EuBaTiO remained to be antiferromagnetic with a reduced transition temperature of 3.1 K, but co-doping of Co/Ni 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 Eu/Eu, 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.