Michelle R. Dolgos

Morphotropic Phase Boundary in the Pb‐Free (1 − x)BiTi3/8Fe2/8Mg3/8O3–xCaTiO3 System: Tetragonal Polarization and Enhanced Electromechanical Properties

Lead-free bismuth-based perovskite oxides with polarization directed along the [001](p) primitive perovskite unit cell edge, analogous to tetragonal PbTiO3, are synthesized at ambient pressure. Enhanced piezoelectric properties, large polarizations, and high depolarization temperatures are observed in the wide morphotropic phase boundary region formed with a rhombohedral phase, with up to 92.5% Bi on the perovskite A site.

Chemical control of octahedral tilting and off-axis A cation displacement allows ferroelectric switching in a bismuth-based perovskite

Bi(Fe2/8Mg3/8Ti3/8)O3 (BFTM) is a member of a small class of pure Bi3+ A site perovskites which are stable without recourse to high pressure synthesis. BFTM is polar in the R3c space group, but ferroelectric switching of the polarisation is not attainable in bulk ceramics. Formation of solid solutions with BaTiO3 produces enhanced functional behaviour. The composition 0.75Bi(Fe2/8Mg3/8Ti3/8)O3–0.25BaTiO3 displays ferroelectric hysteresis loops and piezoelectric response (high field d33 of 85 pC N−1 at 0.1 Hz and low field d33 of 16 pC N−1). This change in functional behaviour is associated with significant changes in the average structure, where the rhombohedral distortion is reduced and transformed to a pseudo-cubic R3m space group, as substitution of the larger Ba2+ cation suppresses tilting of the BO6 octahedra. Polar Bi displacements are refined solely along the pseudocubic p direction of the perovskite subcell, with the off-axis displacements characteristic of BFTM being suppressed. The local structure deviates from the average structure in a similar way to PZT as shown by diffuse scattering in selected area electron diffraction, suggesting correlated p displacements along directions other than the average rhombohedral unique axis. The switchable polarisation measured by PUND measurements is considerably smaller than the remanence measured in P(E) loops.

Synthesis and Solid-State Structural Characterization of a Series of Aqueous Heterometallic Tridecameric Group 13 Clusters

The synthesis and solid-state characterization of a complete series of new heterometallic aqueous nanoscale Ga/In tridecameric clusters is presented. These hydroxo-aquo species significantly expand the library of discrete, aqueous group 13 clusters. This report details the synthetic and structural characterization of these compounds, which are of interest as precursors (inks) for thin-film oxides with materials applications. Single-crystal X-ray diffraction (XRD) data show that the hexagonal unit cell lengths of these clusters fall within the range a, b = 20.134-20.694 Å and c = 18.266-18.490 Å. The unit cell volumes become larger (V = 6494-6774 A(3)) with increasing indium occupancy. The compositions of several Ga/In clusters determined by electron probe microanalysis and elemental analysis are in agreement with single-crystal XRD results. The transformation of the Ga/In clusters to metal oxides at high temperature was studied using variable-temperature powder XRD. With heating, the Ga/In clusters with lower indium occupancies convert to the β-Ga2O3 structure. For clusters with higher indium occupancies, phase separation occurs, and an In2O3 bixbyite-type structure forms. The stoichiometric control at the molecular level demonstrated herein is important in designing functional thin films of metal oxides due to the tunable nature of these heterometallic solution precursors. In addition, information about the solid-state structure of these compounds leads to a fundamental understanding of the materials properties of these clusters for future thin-film and precursor development.

Perovskite B-Site Compositional Control of [110]p Polar Displacement Coupling in an Ambient-Pressure-Stable Bismuth-based Ferroelectric†

Piezoelectrics are key functional materials in actuator and sensor applications.1 Current technology exploits lead-based materials displaying a morphotropic phase boundary (MPB) between two ferroelectric solid solutions of different symmetries and polarization directions. This is best exemplified by the PbZr1−xTixO3 (PZT) perovskite system where the distortions driving the piezo response are produced by the stereo-active electron lone pair of the Pb2+ cation on the A site of the ABO3 perovskite structure.2 Due to the environmental impact of lead, there is a considerable focus on the synthesis of lead-free electroceramics. It is not however clear if the complex crystal chemistry underlying the structural phase boundaries at the MPB in lead-based systems can be simply translated into lead-free analogues. In particular the balance between distortions driven by the A and B site cations and the role of octahedral tilting and tolerance factor considerations are expected to be quite different in non-lead systems, as Pb2+ is significantly larger than the more highly charged Bi3+ often considered as an alternative polarization-generating cation.3 However, the smaller size of bismuth ions generally requires high-pressure synthesis conditions to form a perovskite. Only a small group of known perovskites exist with A-site bismuth cations that are stable at ambient pressure, based on BiFeO3,4 Bi2Mn4/3Ni2/3O6,5 and Bi(Fe2/8Ti3/8Mg3/8)O3 (BFTM).6 Bi2Mn4/3Ni2/3O6 is antiferrodisplacive,5 while both BiFeO3 and BFTM adopt the R′ R3c structure where the [111]p displacements of untilted R (space group R3m in the PZT case) are coupled with octahedral rotation about the same axis (the prime symbol denotes the presence of tilts). In order to access MPBs, polar structures with distinct polarization directions away from [111]p in these Bi-based families are required.

Low temperature synthesis route and structural characterization of (Bi0.5A0.5)(Sc0.5Nb0.5)O3 (A = K+ and Na+) perovskites

Two new perovskites, (Bi0.5A0.5)(Sc0.5Nb0.5)O3 where A = K+ and Na+, have been made phase pure for the first time. The synthesis procedure that prevented the formation of pyrochlore impurities is described. B-site ordering in A = K+ is successfully achieved by prolonged heating. Combined Rietveld refinement on synchrotron X-ray diffraction and neutron diffraction data is used to determine the complex average structure of these materials. A = Na+ crystallizes in the tetragonal P4/mbm space group and has disordered a0a0c+ rotations of BO6 octahedra. A = K+ crystallizes in the cubic Pmm space group, but shows local A-site displacements in the [100] direction. Both materials are highly disordered but illustrate the potential of (A)NbO3-BiScO3 solid solutions as useful piezoelectrics or relaxors.