Abstract
The preferences of the end members of the lanthanide series (Er3+, Tm3+, Yb3+ and Lu3+) for Bi and Fe sites in BiFeO3 were investigated in this paper using X-ray powder diffraction. It was shown that: (1) the crystal structure of the BiFeO3-derived solid solutions remains rhombohedral in the Ln3+ concentration range from 0.01 to 0.1; (2) for the Er3+-, Tm3+- and Yb3+-doped materials, the unit cell parameters of solid solutions (Bi1-xLnxFeO3 or BiFe1-xLnxO3, x = 0–0.1) decrease with increasing x regardless of the selected stoichiometry; (3) for the materials with the same x, the unit cell parameters increase across the Er3+ - Tm3+ - Yb3+ series despite the decrease in ionic radius that occurs across the lanthanide series; and (4) for the Lu3+-doped materials, the unit cell parameters show only a minor increase. These results suggest that with the decreasing ionic radius of the lanthanides, simultaneous doping on the A and B sites corresponding to the formula Bi1-x+zLnх-zFe1-zLnzO3 (Tm3+, Yb3+) gradually replaces dominant A-site doping (Er3+) and eventually finishes with B-site doping (Lu3+). Variations in the magnetic properties of the doped samples indirectly confirm systematic changes in the preferences of the end members of the lanthanide series for Fe sites in BiFeO3: the values of Ms and Mr decrease drastically with the decreasing size of the Ln3+ cation to the extent that the hysteresis loops for the Lu3+-containing samples have a narrow and lenticular shape, which may be due to the dilution of the Fe-sublattice by Lu3+ cations. The samples show a piezoelectric coefficient d33 of 15 pC/N on average and up to 30 pC/N in individual specimens. Despite belonging to a class of multiferroics, Ln3+-containing solid solutions of BiFeO3 show no magnetoelectric or magnetodielectric response due to the weak coupling between piezoelectric and magnetic subsystems.