Intrinsic and tunable ferromagnetism in Bi0.5Na0.5TiO3 through CaFeO3-δ modification
Nguyen Thanh Hung, Nguyễn Hữu Lâm, Anh-Duy Nguyen, Luong Huu Bac, Nguyen Ngoc Trung, Đặng Đức Dũng, Youn Sang Kim, Namsrai Tsogbadrakh, T. Ochirkhuyag, Dorj Odkhuu
Abstract
Abstract New (1- x )Bi 0.5 Na 0.5 TiO 3 + x CaFeO 3-δ solid solution compounds were fabricated using a sol–gel method. The CaFeO 3- δ materials were mixed into host Bi 0.5 Na 0.5 TiO 3 materials to form a solid solution that exhibited similar crystal symmetry to those of Bi 0.5 Na 0.5 TiO 3 phases. The random distribution of Ca and Fe cations in the Bi 0.5 Na 0.5 TiO 3 crystals resulted in a distorted structure. The optical band gaps decreased from 3.11 eV for the pure Bi 0.5 Na 0.5 TiO 3 samples to 2.34 eV for the 9 mol% CaFeO 3-δ -modified Bi 0.5 Na 0.5 TiO 3 samples. Moreover, the Bi 0.5 Na 0.5 TiO 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 0.5 Na 0.5 TiO 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 0.5 Na 0.5 TiO 3 to facilitate the development of lead-free ferroelectric materials.