Iasmi Sterianou

High-temperature (1−x)BiSc1∕2Fe1∕2O3-xPbTiO3 piezoelectric ceramics

Perovskite-structured ceramics in the (1−x)BiSc1∕2Fe1∕2O3-xPbTiO3 (BSF-PT) system were fabricated for x⩾0.45 in which a morphotropic phase boundary (MPB) occurred between rhombohedral and tetragonal phases at x≈0.50(TC=440°C). Samples close to the MPB gave piezoelectric and electromechanical coupling coefficients of ∼300pC∕N and 0.5, respectively. The low cost of BSF-PT in comparison to (1−x)BiScO3-xPbTiO3 (BS-PT) coupled with its high TC and usable piezoelectric properties suggests future commercial exploitation.

Investigation of high Curie temperature (1−x)BiSc1−yFeyO3–xPbTiO3 piezoelectric ceramics

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.

Local stabilisation of polar order at charged antiphase boundaries in antiferroelectric (Bi0.85Nd0.15)(Ti0.1Fe0.9)O3

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.

Defect chemistry of Ti-doped antiferroelectric Bi0.85Nd0.15FeO3

Aberration corrected scanning transmission electron microscopy revealed that Bi0.85Nd0.15Fe0.9Ti0.1O3 ceramics contain coherent Nd-rich precipitates distributed throughout the perovskite lattice, implying charge compensation is obtained by the creation of VNd/// and not VBi///. At low concentrations, therefore, Ti4+ replace Fe2+ with the creation of 2/3VNd///, and at higher concentrations (when Fe2+ have been eliminated and the conductivity suppressed), Fe3+ with the creation of 1/3VNd///. The switch in ionic compensation mechanism from 2/3VNd/// at low Ti concentrations (∼1%) to 1/3VNd/// at higher concentrations (>1%) results in a decrease in the magnitude of ΔTC/Δx, as the disruption of long range anti-polar coupling declines.

Structural phase transitions in Ti-doped Bi1-xNdxFeO3 ceramics

Recently, it was demonstrated that donor doping with Ti on the B-site significantly reduces the conductivity in Bi0.85Nd0.15FeO3 ceramics [Kalantari et al., Adv. Funct. Mater. 21, 3737 (2011)]. In this contribution, the phase transitions as a function of Nd concentration are investigated in 3% Ti doped Bi1-xNdxFeO3 ceramics. Paraelectric (PE) to ferroelectric (FE) transitions were observed for compositions with x ≤ 0.125 which manifested themselves as peaks in permittivity. In contrast, PE to antiferroelectric (AFE) transitions for 0.15 ≤ x ≤ 0.20 gave rise to a step-like change in the permittivity with x = 0.25 exhibiting no sharp anomalies and remaining PE until room temperature. The large volume change at the PE to FE/AFE transitions, reported by Levin and co-workers [Phys. Rev. B 81, 020103 (2011)] and observed here by dilatometry, coupled with their first-order character constrain the transitions to occur uniformly throughout the material in an avalanche-like manner. Hence, the anomalies in DSC, permittivity and thermal expansion occur over a commensurately narrow temperature interval. However, despite the large volume change and eye-catching anomalies in DSC, the latent heats for the transitions in Ti-doped Bi1-xNdxFeO3 are similar to Pb(Zr,Ti)O3 (1–3 kJ/mol) with each an order of magnitude greater than BaTiO3 (∼0.2 kJ/mol). A broad frequency dependent dielectric anomaly of unknown origin in the temperature range 250–450 °C was also observed in all samples.

Phase transitions, domain structure, and pseudosymmetry in La- and Ti-doped BiFeO3

The phase transitions and domain structure of the promising PbO-free solid solution series, (0.95-x)BiFeO3-xLaFeO3-0.05La2/3TiO3, were investigated. X ray diffraction(XRD) revealed a transition from a ferroelectricR3c to a PbZrO3-like (Pbam) antiferroelectric (AFE) structure at x = 0.15 followed by a transition to a paraelectric (PE, Pnma) phase at x > 0.30. The ferroelastic/ferroelectric twin domain width decreased to 10–20 nm with increasing x as the AFE phase boundary was approached but coherent antiphase tilted domains were an order of magnitude greater. This domain structure suggested the local symmetry (20 nm) is lower than the average structure (R3c, a−a−a−) of the tilted regions. The PE phase (x = 0.35) exhibited a dominant a−a−c+ tilt system with Pnma symmetry but diffuse reflections at ∼1/4{ooe} positions suggest that short range antipolar order is residual in the PE phase. The complex domain structure and phase assemblage of this system challenge the conventional interpretation of phase transitions based on macroscopic symmetry. Instead, it supports the notion that frustration driven by chemical distributions at the nanometric level influences the local or pseudo-symmetry as well as the domain structure, with XRD giving only the average macroscopic structure.

Magnetic, ferroelectric, and dielectric properties of Bi(Sc0.5Fe0.5)O3–PbTiO3 thin films

Bi(Sc0.5Fe0.5)O3–PbTiO3 (BSF-PT) thin films with a composition in the vicinity of a morphotropic phase boundary (MPB) between rhombohedral and tetragonal phases have been grown on LaNiO3/SiO2/Si substrates at 550 °C and 150 mTorr by pulsed laser deposition. The dielectric properties of the film were enhanced due to high crystallinity, low porosity, and the vicinity of the MPB. A saturated ferroelectric hysteresis loop was obtained with 2Pr=76 μC/cm, and the leakage current was minimized by cooling the samples, postdeposition in O2. Magnetic measurements revealed that BSF-PT thin films were antiferromagnetic confirming their anticipated multiferroic nature.

Band gap evolution and a piezoelectric-to-electrostrictive crossover in (1 − x)KNbO3–x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 ceramics

The band gap of (1 − x)KNbO3–x(Ba0.5Bi0.5)(Nb0.5Zn0.5)O3 (0 ≤ x ≤ 0.25) ceramics narrows slightly from 3.22 eV for x = 0 to 2.89 eV for x = 0.25, in broad agreement with first-principles calculations [Phys. Rev. B, 2014, 89, 235105]. In addition, an unreported piezoelectric-to-electrostrictive crossover is observed in this compositional range, which is accompanied by a continuous decrease of the maximum electric field-induced strain due to the presence of a non-ferroelectric phase. An electrostriction coefficient of 0.023 m4 C−2 is measured for x = 0.05, whilst no electromechanical response is observed for non-ferroelectric x = 0.25, even under an applied electric field of 80 kV cm−1.

The effect of Li-substitution on the M-phases of AgNbO3

Ceramics in the solid solution, LixAg1−xNbO3 (LAN) with 0 ≤ x ≤ 0.20, have been synthesized by the mixed oxide method. X-ray diffraction (XRD) revealed that only compositions with x ≤ 0.05 were single phase which were indexed according to Pbcm symmetry with a unit cell, √2ap × √2ap × 4ap (where ap is the fundamental perovskite lattice parameter). For compositions with x ≥ 0.1, second phase peaks attributed to LiNbO3 were observed and the matrix phase transformed to a metrically rhombohedral structure according to XRD. A phase transition at 0.05 < x ≤ 0.1 was confirmed by Raman spectroscopy and electron diffraction but the latter technique revealed superstructure reflections for x ≥ 0.1 at ±1/6{00l} and 1/2{ooo} ± 1/6{00l} (where o = odd). These support the premise that Li doping invokes a new complex modulated tilt system with a √2ap × √2ap × 6ap unit cell rather than √2ap × √2ap × 4ap (Pbcm). Reflections of the type ±1/6{00l} are, however, streaked along the [001] direction indicating that the structure may be incommensurate or only partially ordered. The appearance of the √2ap × √2ap × 6ap structure is associated with strong asymmetry in the peak in permittivity attributed to the M3–M2 and a decrease in intensity of the peak in permittivity associated with the M2–M1 transition. Slim polarization versus field curves characteristic of weak ferroelectricity were observed for compositions with x ≤ 0.05 but for higher values of x the loops were unsaturated albeit with larger remanent polarization.Ceramics in the solid solution, LixAg1−xNbO3 (LAN) with 0 ≤ x ≤ 0.20, have been synthesized by the mixed oxide method. X-ray diffraction (XRD) revealed that only compositions with x ≤ 0.05 were single phase which were indexed according to Pbcm symmetry with a unit cell, √2ap × √2ap × 4ap (where ap is the fundamental perovskite lattice parameter). For compositions with x ≥ 0.1, second phase peaks attributed to LiNbO3 were observed and the matrix phase transformed to a metrically rhombohedral structure according to XRD. A phase transition at 0.05 < x ≤ 0.1 was confirmed by Raman spectroscopy and electron diffraction but the latter technique revealed superstructure reflections for x ≥ 0.1 at ±1/6{00l} and 1/2{ooo} ± 1/6{00l} (where o = odd). These support the premise that Li doping invokes a new complex modulated tilt system with a √2ap × √2ap × 6ap unit cell rather than √2ap × √2ap × 4ap (Pbcm). Reflections of the type ±1/6{00l} are, however, streaked along the [001] direction indicating that the structure ...

Multiferroic properties of Bi(Fe0.5Sc0.5)O3–PbTiO3 thin films

Bi(Fe0.5Sc0.5)O3–PbTiO3 (BSF-PT) thin films were grown on LaNiO3/Si and Pt/TiO2/SiO2/Si substrates by pulsed laser deposition. The structure–microstructure–property relations of the films were the investigated using x-ray diffraction and scanning electron microscopy in conjunction with ferroelectric and magnetic measurements. BSF–PT films grown on Pt/TiO2/SiO2/Si substrates show a remnant polarization, Pr>40 μC cm−2, and a coercive field, Ec=1000 kV cm–1. In contrast, films grown under the same deposition conditions on LaNiO3/Si substrates have Pr=20 μC cm−2 and Ec=200 kV cm−2. Moreover, the leakage current density of the film on Pt is reduced by two orders of magnitude from 2×10−4 to 1×10−6 A cm−2. A commensurate improvement in ferromagnetic properties is also observed. Overall, the substitution of Sc for Fe improves the ferroelectric switching behavior and properties and enhances the magnetic moment with respect to BiFeO3–PbTiO3.