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 Bi₇Ti_4−2xCo_xNb_1+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 Bi₇Ti_4−2xCo_xNb_1+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 Bi₇Ti_4−2xCo_xNb_1+xO₂₁ (x = 0.1, 0.2, and 0.3) presents much enhanced properties compared with non-substituted Bi₇Ti₄NbO₂₁. For example, 2E_c = 135.23 kV cm⁻¹ and 2P_r = 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 Bi₇Ti_4−2xCo_xNb_1+xO₂₁ samples have similar values, suggesting that the cobalt atoms in the materials have almost the same efficient moment.