Effect of combined Ca/Ti and Ca/Nb substitution on the crystal and magnetic structure of BiFeO3

Herein, we report on the crystal structure, magnetic and local ferroelectric properties of the Bi1-xCaxFe1-xTixO3 and Bi1-xCaxFe1-x/2Nbx/2O3 perovskites prepared by a solid state reaction method. It has been found that the Ca2+/Nb5+-containing series is characterized by a narrower concentration range (x <= 0.2) over which the acentric R3c structure specific to the pure BiFeO3 can be stabilized. The compositional variation in the critical concentration defining the polar/nonpolar (R3c/Pnma) phase boundary can be understood as related to the chemical modification-induced changes in the lattice spacing diminishing the stability of the a(-) a(-) a(-) filling in favor of the a(-) b(+) a(-) one. Both the Ca2+/Ti4+ and Ca2+/Nb5+ substitutions ensure the suppression of a cycloidal antiferromagnetic order, thus leading to the formation of a weak ferromagnetic polar state. While this effect is proven to be associated with a composition-driven reduction in polar displacements, lattice defects are supposed to contribute to the instability of the cycloidal spin arrangement.