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Bi 1 − x Nd x FeO 3  (0≤x≤0.2) ceramics have been investigated using x-ray diffraction (XRD) and electron diffraction (ED). XRD patterns for x≤0.1 were consistent with rhombohedral BiFeO3, whereas those for x=0.15 and x=0.2 exhibited peak splitting and superlattice reflections representative of orthorhombic, antiferroelectric PbZrO3. ED for the latter samples confirmed the presence of 14(hk0) superlattice reflections typical of a PbZrO3-like structure but additional superlattice reflections were observed at 14(00l) yielding a √2a,2√2a,4a (where a is the pseudocubic lattice parameter) unit cell. The transition from rhombohedral BiFeO3 to the PbZrO3-like structure implies a modification from polar to antipolar behavior.

Observation of an unusual, negatively-charged antiphase boundary in (Bi0.85Nd0.15)(Ti0.1Fe0.9)O3 is reported. Aberration corrected scanning transmission electron microscopy is used to establish the full three dimensional structure of this boundary including O-ion positions to ∼±10 pm. The charged antiphase boundary stabilises tetragonally distorted regions with a strong polar ordering to either side of the boundary, with a characteristic length scale determined by the excess charge trapped at the boundary. Far away from the boundary the crystal relaxes into the well-known Nd-stabilised antiferroelectric phase

In 2009, Karimi et al. reported that Bi1‐xNdxFeO3 0.15 ≤ x ≤ 0.25 exhibited a PbZrO3 (PZ)‐like structure. These authors presented some preliminary electrical data for the PZ‐like composition but noted that the conductivity was too high to obtain radio‐frequency measurements representative of the intrinsic properties. In this study, Bi0.85Nd0.15Fe1‐yTiyO3 (0 ≤ y ≤ 0.1) were investigated, in which Ti acted as a donor dopant on the B‐site. In contrast to the original study of Karimi et al., X‐ray diffraction (XRD) of Bi0.85Nd0.15FeO3 revealed peaks which were attributed to a mixture of PZ‐like and rhombohedral structures. However, as the Ti (0 < y ≤ 0.05) concentration increased, the rhombohedral peaks disappeared and all intensities were attributed to the PZ‐like phase. For y = 0.1, broad XRD peaks indicated a significant decrease in effective diffracting volume. Electron diffraction confirmed that the PZ‐like phase was dominant for y ≤ 0.05, but for y = 0.1, an incommensurate structure was present, consistent with the broadened XRD peaks. The substitution of Fe3+ by Ti4+ decreased the dielectric loss at room temperature from >0.3 to <0.04 for all doped compositions, with a minimum (0.015) observed for y = 0.03. The decrease in dielectric loss was accompanied by a decrease in the room temperature bulk conductivity from ∼1 mS cm−1 to <1 μS cm−1 and an increase in bulk activation energy from 0.29 to >1 eV. Plots of permittivity (ϵr) versus temperature for 0.01 ≤ y ≤ 0.05 revealed a step rather than a peak in ϵr on heating at the same temperature determined for the antiferroelectric–paraelectric phase transition by differential scanning calorimetry. Finally, large electric fields were applied to all doped samples which resulted in a linear dependence of polarisation on the electric field similar to that obtained for PbZrO3 ceramics under equivalent experimental conditions.

Getting the lead out: The bonding preferences of three octahedral‐site cations permit the accommodation of the small, low‐symmetry Bi3+ cation on the twelve‐coordinate A site of the perovskite structure, affording a lead‐free ferroelectric material (see picture). The graph shows the temperature dependence of the relative permittivity (○, left axis) and the dielectric loss (▴, right axis).

Articles you may be interested inStructure, piezoelectric, and ferroelectric properties of BaZrO3 substituted Bi(Mg1/2Ti1/2)O3-PbTiO3 perovskite Perovskite-structured ceramics in the ͑1−x͒BiSc 1/2 Fe 1/2 O 3 -xPbTiO 3 ͑BSF-PT͒ system were fabricated for x ജ 0.45 in which a morphotropic phase boundary ͑MPB͒ occurred between rhombohedral and tetragonal phases at x Ϸ 0.50 ͑T C = 440°C͒. Samples close to the MPB gave piezoelectric and electromechanical coupling coefficients of ϳ300 pC/ N and 0.5, respectively. The low cost of BSF-PT in comparison to ͑1−x͒BiScO 3 -xPbTiO 3 ͑BS-PT͒ coupled with its high T C and usable piezoelectric properties suggests future commercial exploitation.

Ceramics around the morphotropic phase boundary (MPB) in the (1−x)BiSc1−yFeyO3–xPbTiO3 solid solution were fabricated. For y=0.5, ceramics were single phase, and piezoelectric coefficients (d33) and electromechanical coupling coefficients (kp) for MPB compositions were 300 pC/N and 0.49, respectively; a level of piezoelectric activity similar to that of hard, lead zirconate titanate compositions but with TC∼60 °C higher at ∼440 °C. For ceramics with y≥0.7, dielectric measurements in combination with diffraction contrast transmission electron microscopy revealed the existence of two ferroelectric phases for most PbTiO3 contents studied. The presence of two ferroelectric phases was associated with a decrease in piezoelectric activity and although raw materials costs for y=0.7 and 0.8 with respect to y=0 were significantly lower (less Sc2O3) and TC greater (∼500 °C), d33 (∼100 pC/N) and kp (0.18) were too low to be commercially useful for actuator applications.

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