Influence of non-isovalent ion substitution at A site on microstructure and magnetic properties of Ba(Ti0.3Fe0.7)O3 ceramic

The (Ba0.8K0.2)(Ti0.3Fe0.7)O3 ceramic was prepared by solid-state reaction, and post-annealed in oxygen ambient. By comparison with Ba(Ti0.3Fe0.7)O3 made under identical conditions, the effect of non-isovalent A-site substitution of K+ on microstructure and magnetism of as-prepared and annealed Ba(Ti0.3Fe0.7)O3 samples was investigated using X-ray diffraction, Mössbauer spectroscopy, vibrating sample magnetometer and iodometric titration. It is found that all samples have a single 6H-BaTiO3-type hexagonal perovskite structure without any impurities detected, regardless of A-site K+ substitution or annealing. In the as-prepared state, non-isovalent A-site substitution of K+ induces the variation in Fe occupational site, resulting in the disappearance of room-temperature ferromagnetism. The super-exchange interactions of Fe3+ at tetrahedral and octahedral Ti sites determine the paramagnetism of (Ba0.8K0.2)(Ti0.3Fe0.7)O3. During the O2 annealing process, the presence of Fe4+, an unusual valence for iron, besides Fe3+ is observed, both distributed over octahedral Ti site. By A-site substitution of K+ with a lower valence than Ba2+, the charge compensation mechanism is further enhanced, and thus more Fe3+ ions are oxidized to Fe4+ in annealed (Ba0.8K0.2)(Ti0.3Fe0.7)O3. Consequently, the ferromagnetic Fe4+–O2−–Fe4+ super-exchange interactions are strengthened, which leads not only to a paramagnetism–ferromagnetism transition but also to a higher saturation magnetization compared with annealed Ba(Ti0.3Fe0.7)O3.