The structure and properties of Co substituted Bi7Ti4NbO21 with intergrowth phases

Abstract

Multiferroic complex oxides with intergrowth aurivillius phases are gaining more and more attention due to the potential to greatly adjust their ferroelectricity (FE) and ferromagnetism (FM) using non-integer layer numbers. In this work, the 2 + 3 aurivillius intergrowth phases of Bi7Ti4-2xCoxNb1+xO21 were successfully synthesized via a solid reaction method. X-ray diffraction (XRD) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) analyses clearly demonstrated that Co substituted Bi7Ti4-2xCoxNb1+xO21 keeps an intergrowth phase structure when x ≤ 0.3. A new analysis method that maps the linear brightness in HAADF images was used to give the clear Bi atom position, and this revealed that the lattice shrinkage in the c direction caused by Co substitution mainly occurred at the (BiTiNbO7)2- block in the (Bi3TiNbO9) layer, which was also confirmed by an investigation using Raman spectroscopy. Polarization-electric field (P-E) investigations and pulsed polarization positive-up negative-down (PUND) measurements indicated that Bi7Ti4-2xCoxNb1+xO21 (x = 0.1, 0.2, and 0.3) presents much enhanced properties compared with non-substituted Bi7Ti4NbO21. For example, 2Ec = 135.23 kV cm-1 and 2Pr = 9.33 μC cm-2 can be achieved when x = 0.3. Also with Co substitution, Bi7Ti4NbO21 changed from diamagnetic (χ < 0) to paramagnetic (χ ≈ 7 × 10-5). The calculated effective magnetic moments in the Bi7Ti4-2xCoxNb1+xO21 samples have similar values, suggesting that the cobalt atoms in the materials have almost the same efficient moment.