Khomchenko, V.A.; Karpinsky, D.V.; Ivanov, M.S.; Franz, A.; Dubkov, S.V.; Silibin, M.V.; Paixão, J.A.: Effect of combined Ca/Ti and Ca/Nb substitution on the crystal and magnetic structure of BiFeO3. Journal of Magnetism and Magnetic Materials 491 (2019), p. 165561/1-5
10.1016/j.jmmm.2019.165561

Abstract:
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− tilting 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.