A detailed investigation using variable temperature powder neutron diffraction demonstrates that BiFeO 3 undergoes a phase transition from the ferroelectric α phase

Bismuth ferrite suffers from high leakage currents and the presence of a complex incommensurate spin cycloidal magnetic ordering, which has limited its commercial viability and has led researchers to investigate the functionality of doped BiFeO3 ceramics. In particular, the substitution of rare earths onto the Bi(3+) site of the perovskite lattice have been shown to lead to improved functional properties, including lower leakage currents and the suppression of the magnetic spin cycloid. There is particular interest in materials with compositions close to structural morphotropic phase boundaries, because these may lead to materials with enhanced electronic and magnetic properties analogous to the highly relevant PbZrO3- PbTiO3 solid solution. However, many contradictory crystal structures and physical behaviors are reported within the literature. To understand the structure-property relationships in these materials, it is vital that we first unravel the complex structural phase diagrams. We report here a comprehensive review of structural phase transitions in rare-earth-doped bismuth ferrite ceramics across the entire lanthanide series. We attempt to rationalize the literature in terms of the perovskite tool kit and propose an updated phase diagram based on an interpretation of the literature.

The combination of multinuclear solid-state NMR and powder X-ray diffraction has been applied to characterize the octahedron-shaped crystalline nanoparticle products resulting from an inverse micelle synthesis. Rietveld refinements of the powder X-ray diffraction data from the nanoparticles reveal their general formula to be (H 3 O)Y 3 F 10 • xH 2 O. 1 H magic-angle spinning (MAS) NMR experiments provide information on sample purity, as well as serving as an excellent probe of the zeolithic incorporation of atmospheric water. 19 F MAS NMR experiments on a series of monodisperse nanoparticle samples of various sizes yield spectra featuring three unique 19 F resonances, arising from three different fluorine sites within the (H 3 O)Y 3 F 10 • xH 2 O crystal structure. Partial removal of zeolithic water from the internal cavities and tunnels of the nanoparticles leads to changes in the integrated peak intensities in the 19 F MAS NMR spectra; the origin of this behaviour is discussed in terms of 19 F longitudinal relaxation. 19 F-89 Y variable-amplitude cross-polarization (VACP) NMR experiments on both stationary samples and samples under conditions of MAS indicate that two distinct yttrium environments are present, and based on the relative peak intensities, the populations of one of the two sites is closely linked to nanoparticle size. Both 19 F MAS and 19 F-89 Y VACP/MAS experiments indicate small amounts of an impurity present in certain nanoparticles; these are postulated to be spherical amorphous YF 3 nanoparticles. We discuss the importance of probing molecular-level structure in addition to microscopic structure, and how the combination of these characterization methods is crucial for understanding nanoparticle design, synthesis, and application.

High‐temperature powder neutron diffraction experiments are conducted around the reported β–γ phase transition (∼930 °C) in BiFeO3. The results demonstrate that while a small volume contraction is observed at the transition temperature, consistent with an insulator–metal transition, both the β‐ and γ‐phase of BiFeO3 exhibit orthorhombic symmetry; i.e., no further increase of symmetry occurs during this transition. The γ‐orthorhombic phase is observed to persist up to a temperature of approximately 950 °C before complete decomposition into Bi2Fe4O9 (and liquid Bi2O3), which subsequently begins to decompose at approximately 960 °C.

A new family of relaxor dielectrics with the tetragonal tungsten bronze structure (nominal composition Ba 6 M 3+ Nb 9 O 30 , M 3+ = Ga, Sc or In) were studied using dielectric spectroscopy to probe the dynamic dipole response and correlate this with the crystal structure as determined from powder neutron diffraction. Independent analyses of real and imaginary parts of the complex dielectric function were used to determine characteristic temperature parameters, T VF , and T UDR , respectively. In each composition both these temperatures correlated with the temperature of maximum crystallographic strain, T c/a determined from diffraction data. The overall behaviour is consistent with dipole freezing and the data indicate that the dipole stability increases with increasing M 3+ cation size as a result of increased tetragonality of the unit cell. Crystallographic data suggests that these materials are uniaxial relaxors with the dipole moment predominantly restricted to the B1 cation site in the structure. Possible origins of the relaxor behaviour are discussed.