238 research outputs found

    Electron diffraction of tilted perovskites

    Get PDF
    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

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

    Get PDF
    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

    Local stabilisation of polar order at charged antiphase boundaries in antiferroelectric (Bi<sub>0.85</sub>Nd<sub>0.15</sub>)(Ti<sub>0.1</sub>Fe<sub>0.9</sub>)O<sub>3</sub>

    Get PDF
    Observation of an unusual, negatively-charged antiphase boundary in (Bi&lt;sub&gt;0.85&lt;/sub&gt;Nd&lt;sub&gt;0.15&lt;/sub&gt;)(Ti&lt;sub&gt;0.1&lt;/sub&gt;Fe&lt;sub&gt;0.9&lt;/sub&gt;)O&lt;sub&gt;3&lt;/sub&gt; 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

    Maghemite-like regions at crossing of two antiphase boundaries in doped BiFeO3

    Get PDF
    We report the observation of a novel structure at the point where two antiphase boundaries cross in a doped bismuth ferrite of composition (Bi0.85Nd0.15)(Fe0.9Ti0.1)O0.3. The structure was investigated using a combination of high angle annular dark field imaging and electron energy loss spectroscopy spectrum imaging in the scanning transmission electron microscope. A three-dimensional model was constructed by combining the position and chemistry data with previous results and assuming octahedral coordination of all Fe and Ti atoms. The resulting structure shows some novel L shaped arrangements of iron columns, which are coordinated in a similar manner to FeO6 octahedra in maghemite. It is suggested that this may lead to local ferromagnetic orderings similar to those in maghemite

    Novel nanorod precipitate formation in neodymium and titanium codoped bismuth ferrite

    Get PDF
    The discovery of unusual nanorod precipitates in bismuth ferrite doped with Nd and Ti is reported. The atomic structure and chemistry of the nanorods are determined using a combination of high angle annular dark field imaging, electron energy loss spectroscopy, and density functional calculations. It is found that the structure of the BiFeO3 matrix is strongly modified adjacent to the precipitates; the readiness of BiFeO3 to adopt different structural allotropes in turn explains why such a large axial ratio, uncommon in precipitates, is stabilized. In addition, a correlation is found between the alignment of the rods and the orientation of ferroelastic domains in the matrix, which is consistent with the system's attempt to minimize its internal strain. Density functional calculations indicate a finite density of electronic states at the Fermi energy within the rods, suggesting enhanced electrical conductivity along the rod axes, and motivating future investigations of nanorod functionalities

    Origin of improved tunability and loss in N2 annealed barium strontium titanate films

    Get PDF
    Barium strontium titanate (BSTO) thin films were deposited on Pt(111) by high throughput evaporative physical vapor deposition and then annealed at 650 °C for 30 min under N2 atmosphere. Using advanced transmission electron microscopy, energy-dispersive x-ray spectroscopy and electron energy-loss spectroscopy, we directly show that not only does N substitute for O in the BSTO lattice but that it also compensates for Ti3+ ions, suppressing conductivity, thereby reducing dielectric loss and enhancing dielectric tunability. However, this effect is negated near the film edge where we speculate that exposed Pt acts as a reservoir of adsorbed/absorbed O and alters the local N2 concentration during annealing

    Designing pseudocubic perovskites with enhanced nanoscale polarization

    Get PDF
    A crystal-chemical framework has been proposed for the design of pseudocubic perovskites with nanoscale ferroelectric order, and its applicability has been demonstrated using a series of represen- tative solid solutions that combined ferroelectric (K 0.5 Bi 0.5 TiO 3 , BaTiO 3 , and PbTiO 3 ) and antifer- roelectric (Nd-substituted BiFeO 3 ) end members. The pseudocubic structures obtained in these systems exhibited distortions that were coherent on a scale ranging from sub-nanometer to tens of nanometers, but, in all cases, the macroscopic distortion remained unresolvable even if using high- resolution X-ray powder diffraction. Different coherence lengths for the local atomic displacements account for the distinctly different dielectric, ferroelectric, and electromechanical properties exhib- ited by the samples. The guidelines identified provide a rationale for chemically tuning the coher- ence length to obtain the desired functional response

    The effect of substrate clamping on the paraelectric to antiferroelectric phase transition in Nd-doped BiFeO₃ thin films

    Get PDF
    Thin films were deposited on Pt/Ti/SiO₂/Si substrates using pulsed laser deposition from a target with a composition (Bi₀.₈₂₅Nd₀.₁₇₅Fe₀.₉₇Ti₀.₀₃O₃) with 5 mol% excess Bi₂O₃ within the antiferroelectric (AFE) region of the NdFeO₃-BiFeO₃ phase diagram. However, Raman spectroscopy and transmission electron microscopy (TEM) revealed that films consisted of a mosaic microstructure in which (AFE), ferroelectric (FE) and paraelectric (PE) phases coexisted. Variation in the spatial distribution of Nd is typically greater in bulk ceramics than in thin films and therefore, the absence of single phase AFE cannot be attributed to local changes in composition. Instead, it is proposed that clamping due to mismatch in thermal expansion coefficient with the substrate suppresses the large volume change associated with the PE-FE and PE-AFE transition in bulk and its absence in the thin film prevents an avalanche-like transition throughout grains, which in bulk sustains single phase AFE, irrespective of local deviations in the Nd concentration

    A resource efficient design strategy to optimise the temperature coefficient of capacitance of BaTiO3-based ceramics using finite element

    Get PDF
    An in-house finite element modelling package is used to simulate the electrical response of core–shell microstructures of BaTiO3-based (BT) dielectric materials as a design strategy for multilayer ceramic capacitors. By combining this method with experimental material properties, both the temperature coefficient of capacitance (TCC) and the volume ratio of undoped BT ceramic (core phase) to chemically doped BT ceramic (shell phase) to optimise temperature stability can be predicted. This is a faster, more reliable and efficient design strategy than current techniques which are based on iterative experimental protocols. This methodology is illustrated using rare earth-free NaNbO3-doped BT (NNBT) ceramics as an example for which we predict and experimentally confirm an optimised volume fraction ratio of 0.66 undoped BT core to 0.34 of a 2.5% NNBT shell
    corecore