Intrinsic and tunable ferromagnetism in Bi _0.5Na _0.5TiO ₃ through CaFeO _3-δ modification

Abstract

New (1- x)Bi _0.5Na _0.5TiO ₃ +  xCaFeO _3-δ solid solution compounds were fabricated using a sol–gel method. The CaFeO _{3- δ} materials were mixed into host Bi _0.5Na _0.5TiO ₃ materials to form a solid solution that exhibited similar crystal symmetry to those of Bi _0.5Na _0.5TiO ₃ phases. The random distribution of Ca and Fe cations in the Bi _0.5Na _0.5TiO ₃ crystals resulted in a distorted structure. The optical band gaps decreased from 3.11 eV for the pure Bi _0.5Na _0.5TiO ₃ samples to 2.34 eV for the 9 mol% CaFeO _3-δ-modified Bi _0.5Na _0.5TiO ₃ samples. Moreover, the Bi _0.5Na _0.5TiO ₃ samples exhibited weak photoluminescence because of the intrinsic defects and suppressed photoluminescence with increasing CaFeO _3-δ concentration. Experimental and theoretical studies via density functional theory calculations showed that pure Bi _0.5Na _0.5TiO ₃ exhibited intrinsic ferromagnetism, which is associated with the possible presence of Bi, Na, and Ti vacancies and Ti ³⁺-defect states. Further studies showed that such an induced magnetism by intrinsic defects can also be enhanced effectively with CaFeO _3-δ addition. This study provides a basis for understanding the role of secondary phase as a solid solution in Bi _0.5Na _0.5TiO ₃ to facilitate the development of lead-free ferroelectric materials.