David I. Woodward

Crystal and domain structure of the BiFeO3–PbTiO3 solid solution

X-ray diffraction and transmission electron microscopy have been performed on samples in the solid solution series (BiFeO3)x–(PbTiO3)1−x in which a morphotropic phase boundary occurs at x≈0.7. BiFeO3 exhibits superlattice reflections at 12{hkl}p positions in some electron diffraction patterns, the distribution of which unambiguously demonstrates that the FeO6 octahedra are rotated in anti-phase about the pseudocubic [111] axis, consistent with the rhombohedral (R) space group R3c. The amplitude of the rotations decreases in the R phase as PbTiO3 content increases and superlattice reflections are absent in electron diffraction patterns from the tetragonal (T) phase (x=0.6), indicating that it is untilted with space group P4mm. Electron diffraction patterns from samples where x=0.7 reveal superlattice reflections not associated with octahedral rotations and consistent with an intermediate phase with lower symmetry than T and R.

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.

Investigation of a high Tc piezoelectric system: (1−x)Bi(Mg1/2Ti1/2)O3–(x)PbTiO3

(1−x)Bi(Mg1/2Ti1/2)O3–xPbTiO3 polycrystalline ceramics were investigated for potential as high-temperature piezoelectric materials. A morphotropic phase boundary (MPB) between tetragonal (T) and rhombohedral (R) ferroelectric (FE) phases, which exhibited enhanced piezoelectric activity and a ferroelectric–paraelectric phase transition at 478 °C was observed at x≈0.37. Electron diffraction patterns (x⩽0.37) contained discrete superlattice reflections at 12{hkl} positions arising from antiphase rotations of the O octahedra, consistent with R3c space group symmetry. These reflections were diffuse at the MPB (x=0.38) and absent in the T phase (x=0.5). In the unpoled state, FE R (x=0.35) ceramics revealed a polar microdomain structure whereas the T phase (x=0.5) contained classic {110} twin domain boundaries. However, poled R samples underwent a field-induced transformation to an aligned domain structure with {110} twin boundaries similar to those in the T phase. Correlations are made between structure and pro...

Coupling between octahedral tilting and ferroelectric order in tetragonal tungsten bronze-structured dielectrics

Strong coupling between local polar displacements and a commensurate octahedral tilting is proposed to explain the onset of classic ferroelectric behavior in tetragonal tungsten bronzelike dielectrics Ba2LaxNd1−xNb3Ti2O15. The ferroelectric phase transition is associated with a discontinuous non-lock-in transformation of an incommensurate tilted structure to a commensurate superstructure. In a manner reminiscent of perovskitelike oxides, the driving force for commensurate tilting increases as the average ionic radius of the rare-earth ion decreases; no classical ferroelectric transition is observed for compositions with x>0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.

Dielectric and structural studies of Ba2MTi2Nb3O15 (BMTNO15, M=Bi3+,La3+,Nd3+,Sm3+,Gd3+) tetragonal tungsten bronze-structured ceramics

The structure and dielectric properties of a new family of tetragonal tungsten bronze (TTB) ceramics with the general formula, Ba2MTi2Nb3O15 where M=Bi3+,La3+,Nd3+,Sm3+,Gd3+, have been investigated. Hereafter the compositions will be referred to by the abbreviation BMTNO15 where M=B(Bi3+), L(La3+), N(Nd3+), S(Sm3+) or G(Gd3+). BLTNO15 [permittivity maximum (Tm)∼−80°C] and BBTNO15 (Tm∼−100°C) exhibited relaxorlike dielectric behavior. In contrast, BNTNO15 (Tm∼165°C), BSTNO15 (Tm∼250°C), and BGTNO15 (Tm∼320°C) were classic ferroelectrics. Tm increased with decreasing radius of the M ion. Room temperature x-ray powder diffraction (XRD) patterns of all the compounds indexed on a prototype P4∕mbm (or P4bm) space group with lattice parameters a≈12.4A and c≈4A. However, electron diffraction revealed that the relaxor phases, BLTNO15 and BBTNO15, exhibited an incommensurate modulation, whereas the classic ferroelectric BNTNO15, BSTNO15, and BGTNO15 featured an orthorhombic superstructure with lattice parameters a≈...

Vacancy ordering in reduced barium titanate

A crystal structure is proposed for reduced barium titanate, BaTiO3−δ, δ≈0.33, formed during the degradation of Ni–BaTiO3 X7R multilayer ceramic capacitors. High-resolution transmission electron microscopy and selected-area electron diffraction have been used in combination with computer simulations to show that oxygen vacancies accrete on every third pseudocubic {111} plane, resulting in a cell with space group P3m1. Additionally, from electron energy loss spectroscopy, it is proposed that Ti4+ is reduced to Ti3+ as a mechanism of charge compensation within oxygen-deficient octahedra.

Transmission electron microscopy investigation of the high temperature BiScO3-PbTiO3 piezoelectric ceramic system

Recently, a perovskite solid-solution ((1−x)BiScO3−(x)PbTiO3) with superior electromechanical properties than PbZrO3−PbTiO3 (PZT) ceramics has been discovered. The system has a morphotropic phase boundary (MPB) between ferroelectric rhombohedral (R) and tetragonal (T) symmetries similar to PZT but the underlying domain structures and some aspects of the crystal chemistry remain to be elucidated. Therefore, transmission electron microscopy has been used to study changes in domain structure and symmetry as a function of x. Domain structures in the R and T phases have been found which closely resemble those in PZT. In addition, 12{hkl} superlattice reflections were observed, particularly in the 〈110〉 zone axis diffraction patterns from grains of the R phase (e.g., x=0.5), which were consistent with rotations of the octahedra in antiphase and the space group R3c. These reflections were absent in the R phase close to the MPB (x=0.64) and in the T phase (x=0.75). From these results, the low temperature phase di...

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.

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.

Structural characterisation of ferroelectric Ag2Nb4O11 and dielectric Ag2Ta4O11

Structural characterisation of Ag2Nb4O11 and Ag2Ta4O11 powders has been carried out by Rietveld refinement of neutron powder diffraction data in the temperature range 10–573 K, symmetry-adapted mode analysis and second-harmonic generation. Ag2Nb4O11 ceramics appear to be ferroelectric with a maximum permittivity of ∼150 at TC ≈ 400 K, frequency-independent permittivity below this temperature and a shoulder in the e′ vs. temperature profile at ∼200 K. The space group changes from Rc to R3c as the temperature decreases through TC with a further transition from R3c to R3 below ∼200 K. The origin of the polar structure is related to displacement of Nb atoms in pentagonal bipyramids towards apical oxygens and in octahedra towards faces. This displacement towards octahedra faces is observed for some Ag atoms too. Ag2Ta4O11 ceramics show temperature-independent permittivity of ∼20, with a centrosymmetric crystal structure, Rc. Comments on possible phase transition(s) of several compounds similar to Ag2(Nb,Ta)4O11 are made.