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Density functional theory calculations are conducted for tetragonal BiFeO3–BaTiO3 superlattices to investigate the influence of electronic structures on ferroelectric spontaneous polarization (P s). When the number of the perovskite unit cells in one layer (N) is decreased below 10, the P s starts to decrease from the volume-averaged one (50.9 μC cm−2) and eventually becomes half at N = 1. In the BiFeO3 cell (N = ∞) with a large P s (73.3 μC cm−2), a covalent bond arising from a Bi_6p-O_2p orbital interaction is extended through a Bi–O network and stereo-chemical lone-pair electrons of Bi are accommodated in the opposite direction of the polar c axis. In the superlattice with N = 1, the Bi–O network cannot be developed by the presence of Ba, and then the Bi–O bond becomes ionic. We show that the large P s of the BiFeO3 cell originates from the Bi_6p-O_2p mixing superimposed on the stereo-chemical nature of the lone-pair electrons of Bi.

Ferroelectric BaTiO3 (BT) thin-film capacitors with a buffer layer of (Ba1− x Sr x )TiO3 (BST) have been fabricated on (001) SrTiO3 (STO) single-crystal substrates by a pulsed laser deposition method, and the crystal structure and polarization hysteresis properties have been investigated. X-ray diffraction reciprocal space mapping shows that the BST buffer effectively reduces the misfit strain relaxation of the BT films on SrRuO3 (SRO) electrodes. The BT capacitor with the SRO electrodes on the BST (x = 0.3) buffer exhibits a well-saturated hysteresis loop with a remanent polarization of 29 µC/cm2. The hysteresis loop displays a shift toward a specific field direction, which is suggested to stem from the flexoelectric coupling between the out-of-plane polarization and the strain gradient adjacent to the bottom interface.

Mn doping effect on the crystal structure and spontaneous polarization (Ps) in ferroelectric BiFeO3 (BFO) thin films are experimentally and theoretically investigated. Polarization hysteresis measurements for BiFe1−xMnxO3 (BFMO) films grown on (100) SrTiO3 substrates with SrRuO3 electrodes indicate that remanent polarization (Pr) remains unchanged in the range of 0 ≤ x ≤ 0.1. As a result of the density-functional theory (DFT) calculations for the rhombohedral, tetragonal, and monoclinic phases of BFO and BFMO (x = 0.5), we find that the monoclinic phase with compressive strain from the substrates yields the most reasonable Ps. The DFT calculations also indicate that heavy doping of up to 50% Mn does not lead to a considerable deterioration in Ps, which is consistent with the experiments. Our investigations demonstrate that the ferroelectric polarization in the BFMO system can be retained for a wide range of Mn content either in bulk or thin-film form.

Crystal structures and microstructures associated with ferroelectric properties in a lead-free piezoelectric compound, x(Bi0.5K0.5)TiO3–(1-x)BiFeO3, have been investigated mainly by transmission electron microscopy (TEM). Complicated ferroelectric domain structures with a width of approximately ∼200 nm were found in the polar rhombohedral structure of the x = 0.30 compound. On the other hand, a tweed-like contrast, which stems from local lattice distortion, was observed in the pseudocubic structure of the x = 0.40, 0.46, and 0.50 compounds. The present results suggest that the pseudocubic phase is characterized by nanodomain structures consisting of non-polar cubic and polar rhombohedral structures.