Intrinsic and tunable ferromagnetism in Bi0.5Na0.5TiO3 through CaFeO3-δ modification

N. T. Hung, N. H. Lam, A. D. Nguyen, L. H. Bac, N. N. Trung, D. D. Dung, Y. S. Kim, N. Tsogbadrakh, T. Ochirkhuyag & D. Odkhuu

Abstract

New (1-x) Bi_0.5Na_0.5TiO₃ +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_0.5TiO₃ materials to form a solid solution that exhibited similar crystal symmetry to those of Bi_0.5Na_0.5TiO₃ phases. The random distribution of Ca and Fe cations in the Bi_0.5Na_0.5TiO₃ crystals resulted in a distorted structure. The optical band gaps decreased from 3.11 eV for the pure Bi_0.5Na_0.5TiO₃ samples to 2.34 eV for the 9 mol% CaFeO_3-δ-modified Bi_0.5Na_0.5TiO₃ samples. Moreover, the Bi_0.5Na_0.5TiO₃ 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_0.5TiO₃ exhibited intrinsic ferromagnetism, which is associated with the possible presence of Bi, Na, and Ti vacancies and Ti³⁺-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_0.5TiO₃ to facilitate the development of lead-free ferroelectric materials. (Read More)