Strong smearing and disappearance of phase transitions into polar phases due to inhomogeneous lattice strains induced by A-site doping in Bi(1-x)A(x)FeO(3) (A: La, Sm, Gd)

Official URL: http://dx.doi.org/10.1016/j.jallcom.2014.03.103

Doping of ferroics is often intended to generate new functionalities or enhance the already existing properties, but it comes at the expense of local structural distortions around dopants in the lattice. We report on the effect of A-site doping and their effect on the phase transition temperatures of sol-gel synthesized Bi-1 (x)A(x)FeO(3) (A: Gd, Sm, La) powders as a function of dopant type and concentration. A clear direct correlation between structural parameters and transition temperatures was noted as a function of ionic radii of dopants for any given concentration, implying the effect of inhomogeneous lattice strains around dopants. There is a dramatic reduction in the phase transition temperatures of BiFeO3 upon doping determined with differential thermal analysis. This is accompanied by a partial volume of the grains gradually shifting from the bulk rhombohedral towards a higher symmetry structure particularly in Sm and Gd doped powders while this change is minimal in La doped powders as evidenced by X-ray diffraction and Raman spectroscopy. We find that a phase mixture forms in powders whose fraction is a strong function of dopant radius for a given concentration. Moreover, there is a direct correlation between the ionic radius and the extent of reduction in the transition temperature of the polar phase in the mixture for a given dopant concentration. We suggest a mechanism to explain the inhomogeneous nature of the transition of the sol-gel synthesized powders where the dramatic reduction in the transition temperatures of Sm and Gd doped BiFeO3 is due to local lattice strains around unitcells containing dopant ions that create gradients in polarization leading to internal depolarizing fields, possibly stabilizing non-polar phases. We conclude that local disappearance of stereochemical activity of Bi3+ due to lone pairs is not sufficient to explain the dramatic changes in phase transition temperatures because of strong dependence on ionic radii of dopants.