Modified multiferroic properties in narrow bandgap (1-x)BaTiO₃-xBaNb_1/3Cr_2/3O_3-δ ceramics

Perovskite oxides of formula (1-x)BaTiO₃-xBaNb_1/3Cr_2/3O_3-δ ((1-x)BT-xBNC), a series of lead-free ferroelectric photovoltaic compounds, have been sintered via the solid-state reaction technology and simultaneously show ferromagnetic orders and bandgap narrowing effects at room-temperature (RT). X-ray diffraction patterns reveal the doping-induced transition from tetragonal to cubic phase, which is explained by a possible mechanism. Raman spectroscopy analysis not only further confirms the presence of structural transition, but also indicates the difference in local crystal symmetry compared with the idealized cubic phase. Moreover, the minimum bandgap of as-prepared samples is about 2.1 eV, much smaller than that of the pure BT ceramic (3.1 eV), which is attributed to Cr³⁺ 3d orbits within the highest occupied orbital and the lowest unoccupied orbital of BT. It is noted that the higher doping concentration is conducive to the ferromagnetic order but unfavourable to the ferroelectric polarization. The above results exhibit the abundant physical phenomena and provide a method to design the solar-energy device and the multiferroic information storage application.