Domain variance and superstructure across the antiferroelectric/ferroelectric phase boundary in Pb1−1.5xLax(Zr0.9TiM0.1)O3

Transmission electron microscopy, x-ray diffraction, relative permittivity as a function of temperature, and polarization versus field loops were used to study the antiferroelectric/ferroelectric (AFE/FE) phase boundary in Pb1−1.5xLaxZr0.9Ti0.1O3 (PLZT, 100x/90/10) ceramics. X-ray diffraction and electrical measurements indicated a FE rhombohedral (R) to AFE tetragonal (T) phase transition between PLZT 2/90/10 and 4/90/10. Both phases exhibited superstructure reflections in electron-diffraction patterns at 1⁄2{hkl} positions consistent with rotations of the octahedra in antiphase. Previously, neutron diffraction suggested that the FER has an a−a−a− tilt system (Glazer notation), in agreement with its macroscopic symmetry. By analogy, it is proposed that the AFET phase has an a0a0c− tilt system. The AFE phase was also characterized by incommensurate superstructure along pseudocubic 〈110〉p directions, whereas the FE phase had extra commensurate superlattice reflections at 1⁄2{hk0}p positions. 1⁄2{hk0}p reflections are forbidden in both tilt systems, but their presence is explained by Pb ion displacements averaged along 〈111〉 but with short coherence antiparallel components along 〈110〉 directions. The antiparallel Pb displacements are coupled to an a−b−b− (a ≈ b) monoclinic tilt system in the vicinity of the AFE/FE boundary

Electron diffraction of tilted perovskites

Simulations of electron diffraction patterns for each of the known perovskite tilt systems have been performed. The conditions for the appearance of superlattice reflections arising from rotations of the octahedra are modified to take into account the effects of different tilt systems for kinematical diffraction. The use of selected-area electron diffraction as a tool for perovskite structure determination is reviewed and examples are included

Characterization of high-fracture toughness K-fluorrichterite-fluorapatite glass ceramics

Stoichiometric K-fluorrichterite (Glass A) and the same composition with 2 mol% P2O5 added (Glass B) were prepared and then heat-treated isothermally from 550°1000°C with 50°C intervals. Samples were characterized using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The biaxial flexural strength and indentation fracture toughness of heat-treated glass specimens were also determined for both materials. XRD traces and TEM images showed similar phase evolution and fine microstructures for both systems at ≤950°C, with mica and diopside reacting with residual glass to form K-fluorrichterite as the temperature was increased from 650°C. However, in Glass B, fluorapatite was also present at >800°C. In contrast, coarser microstructures were observed at 1000°C, with larger K-fluorrichterite (20 μm) and enstatite (10 μm) crystals in Glasses A and B, respectively. The highest fracture toughness (2.69 ± 0.01 MPa·m(1/2)) and biaxial strength (242.6 ± 3.6 MPa) were recorded for Glass B heat-treated at 1000°C. This was attributed to the presence of enstatite coupled with an interlocked lath-like crystalline microstructure

Temperature Dependent Piezoelectric Properties of Lead-Free (1-x)K0.6Na0.4NbO3–xBiFeO3 Ceramics

(1-x)K0.4Na0.6NbO3–xBiFeO3 lead-free piezoelectric ceramics were successfully prepared in a single perovskite phase using the conventional solid-state synthesis. Relative permittivity (εr) as a function of temperature indicated that small additions of BiFeO3 not only broadened and lowered the cubic to tetragonal phase transition (TC) but also shifted the tetragonal to orthorhombic phase transition (TO–T) toward room temperature (RT). Ceramics with x = 1 mol.% showed optimum properties with small and large signal piezoelectric coefficient, d33 = 182 pC/N and d∗33 = 250 pm/V, respectively, electromechanical coupling coefficient, kp = 50%, and TC = 355°C. kp varied by ∼5% from RT to 90°C, while d∗33 showed a variation of ∼15% from RT to 75°C, indicating that piezoelectric properties were stable with temperature in the orthorhombic phase field. However, above the onset of TO–T, the properties monotonically degraded in the tetragonal phase field as TC was approached

Terahertz and Infrared Studies of Antiferroelectric Phase Transition in Multiferroic Bi_0.85Nd_0.15FeO_3

High-frequency dielectric studies of Bi_0.85Nd_0.15FeO_3 ceramics performed betweeen 100 and 900 K reveal hardening of most polar phonons on cooling below antiferroelectric phase transition, which occurs near 600 K. Moreover, a strong THz dielectric relaxation is seen in paraelectric phase. Its relaxation frequency softens on cooling towards Tc ~ 600 K, its dielectric strength simultaneously decreases and finaly the relaxation disappears from the spectra below 450 K. Both phonon and dielectric relaxation behavior is responsible for a decrease in the dielectric permittivity at the antiferroelectric phase transition. Origin of unusual strong THz dielectric relaxation in paraelectric phase is discussed. Bi_0.85Nd_0.15FeO_3 structrure lies on the phase boundary between polar rhombohedral and non-polar orthorhombic phase and owing to this the polarization rotation and polarization extension can enhance the piezoelectric response of this system. Similarities and discrepancies with lead-based piezoelectric perovskites, exhibiting morphotrophic phase boundary between two ferroelectric phases, are discussed.Comment: JAP, in prin

HRTEM study of a new non-stoichiometric BaTiO(3-δ) structure

BaTiO3-based multilayer ceramic capacitors (MLCCs) with Ni internal electrodes are co-fired in reducing atmospheres to avoid oxidation of the electrode. Although dielectric materials are doped by acceptor, donor and amphoteric dopants to minimize the oxygen vacancy content, there is still a large concentration of oxygen vacancies that are accommodated in the BaTiO3 active layers. In general, ABO3 perovskites demonstrates a strong ability to accommodate the oxygen vacancies and maintain a regular pseudo-cubic structure. Oxygen deficient barium titanate can be transformed to a hexagonal polymorph (h-BT) at high temperatures1,2. In this paper, we report the new modulated and long range ordered structures of non-stoichiometric BaTiO3-δ that are observed in the electrically degraded Ni-BaTiO3 MLCCs at low temperature

The atomic structure and chemistry of Fe-rich steps on antiphase boundaries in Ti-doped Bi_0.9Nd_0.15FeO3

Stepped antiphase boundaries are frequently observed in Ti-doped Bi<sub>0.85</sub>Nd<sub>0.15</sub>FeO<sub>3</sub>, related to the novel planar antiphase boundaries reported recently. The atomic structure and chemistry of these steps are determined by a combination of high angle annular dark field and bright field scanning transmission electron microscopy imaging, together with electron energy loss spectroscopy. The core of these steps is found to consist of 4 edge-sharing FeO<sub>6</sub> octahedra. The structure is confirmed by image simulations using a frozen phonon multislice approach. The steps are also found to be negatively charged and, like the planar boundaries studied previously, result in polarisation of the surrounding perovskite matrix

Controlling mixed conductivity in Na 1/2 Bi 1/2 TiO 3 using A-site non-stoichiometry and Nb-donor doping

Precise control of electronic and/or ionic conductivity in electroceramics is crucial to achieve the desired functional properties as well as to improve manufacturing practices. We recently reported the conventional piezoelectric material Na1/2Bi1/2TiO3 (NBT) can be tuned into a novel oxide-ion conductor with an oxide-ion transport number (tion) > 0.9 by creating bismuth and oxygen vacancies. A small Bi-excess in the nominal starting composition (Na0.50Bi0.50+xTiO3+3x/2, x = 0.01) or Nb-donor doping (Na0.50Bi0.50Ti1−yNbyO3+y/2, 0.005 ≤ y ≤ 0.030) can reduce significantly the electrical conductivity to create dielectric behaviour by filling oxygen vacancies and suppressing oxide ion conduction (tion ≤ 0.10). Here we show a further increase in the starting Bi-excess content (0.02 ≤ x ≤ 0.10) reintroduces significant levels of oxide-ion conductivity and increases tion ∼ 0.4–0.6 to create mixed ionic/electronic behaviour. The switch from insulating to mixed conducting behaviour for x > 0.01 is linked to the presence of Bi-rich secondary phases and we discuss possible explanations for this effect. Mixed conducting behaviour with tion ∼ 0.5–0.6 can also be achieved with lower levels of Nb-doping (y ∼ 0.003) due to incomplete filling of oxygen vacancies without the presence of secondary phases. NBT can now be compositionally tailored to exhibit three types of electrical behaviour; Type I (oxide-ion conductor); Type II (mixed ionic-electronic conductor); Type III (insulator) and these results reveal an approach to fine-tune tion in NBT from near unity to zero. In addition to developing new oxide-ion and now mixed ionic/electronic NBT-based conductors, this flexibility in control of oxygen vacancies allows fine-tuning of both the dielectric/piezoelectric properties and design manufacturing practices for NBT-based multilayer piezoelectric devices

Timing of therapeutic hypothermia for inborn and outborn infants with neonatal encephalopathy

Therapeutic hypothermia is now the standard of care for infants with moderate to severe hypoxic ischaemic encephalopathy. Sixty-three infants received therapeutic hypothermia at Cork University Maternity Hospital (CUMH) from 2010-2014. Median gestational age was 40 weeks. Eighteen (29%) infants were Sarnat grade 3, 41(65%) grade 2 and 4(6%) grade 1. Nineteen outborn infants arrived in CUMH at a median (IQR) age of 310 (270, 420) minutes. Four (21%) outborn infants were within the target temperature range on arrival. Median (IQR) time (minutes) from birth to achieve target temperature was 136 (90, 195) for inborn and 300 (240, 360) for outborn infants (p <.01). Overall, 35 (56%) infants had electrical seizures, 42 (74%) had a normal MRI at a median (IQR) age of 7(6,9) days and the median(IQR) length of stay was 9 (7,11) days. Although no difference in seizures or MRI findings was seen, passive cooling does not achieve consistent temperature control for outborn infants

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