The available literature on the crystal structure of the metastable alumina polymorphs and their associated transitions is critically reviewed and summarized. All the metastable alumina structures have been identified as ordered or partially ordered cation arrays on the interstitial sites of an approximately close-packed oxygen sublattice (either face-centered cubic or hexagonal close packed). The analysis of the symmetry relations between reported alumina polymorphs having an approximately face-centered cubic packing of the oxygen anions allows for an exact interpretation of all the complex domain structures that have been observed experimentally. Possible mechanisms for the phase transitions between the different alumina polymorphs also are discussed.

Emerging complex functional materials often have atomic order limited to the nanoscale. Examples include nanoparticles, species encapsulated in mesoporous hosts, and bulk crystals with intrinsic nanoscale order. The powerful methods that we have for solving the atomic structure of bulk crystals fail for such materials. Currently, no broadly applicable, quantitative, and robust methods exist to replace crystallography at the nanoscale. We provide an overview of various classes of nanostructured materials and review the methods that are currently used to study their structure. We suggest that successful solutions to these nanostructure problems will involve interactions among researchers from materials science, physics, chemistry, computer science, and applied mathematics, working within a "complex modeling" paradigm that combines theory and experiment in a self-consistent computational framework.

Photoelectrochemical (PEC) water splitting represents a promising route for renewable production of hydrogen, but trade-offs between photoelectrode stability and efficiency have greatly limited the performance of PEC devices. In this work, we employ a metal-insulator-semiconductor (MIS) photoelectrode architecture that allows for stable and efficient water splitting using narrow bandgap semiconductors. Substantial improvement in the performance of Si-based MIS photocathodes is demonstrated through a combination of a high-quality thermal SiO2 layer and the use of bilayer metal catalysts. Scanning probe techniques were used to simultaneously map the photovoltaic and catalytic properties of the MIS surface and reveal the spillover-assisted evolution of hydrogen off the SiO2 surface and lateral photovoltage driven minority carrier transport over distances that can exceed 2 cm. The latter finding is explained by the photo- and electrolyte-induced formation of an inversion channel immediately beneath the SiO2/Si interface. These findings have important implications for further development of MIS photoelectrodes and offer the possibility of highly efficient PEC water splitting.

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Structural differences in the so-called M polymorphs of AgNbO 3 were analyzed using combined highresolution x-ray diffraction, neutron total scattering, electron diffraction, and x-ray absorption fine-structure measurements. These polymorphs all crystallize with Pbcm symmetry and lattice parameters ͱ2a c ϫ ͱ 2a c ϫ 4a c ͑where a c Ϸ 4 Å corresponds to the lattice parameter of an ideal cubic perovskite͒ which are determined by a complex octahedral tilt system ͑a − b − c − ͒ / ͑a − b − c + ͒ involving a sequence of two in-phase and two antiphase rotations around the c axis. Our results revealed that, similar to KNbO 3 , the Nb cations in AgNbO 3 exhibit local off-center displacements correlated along Nb-Nb-Nb chains. The displacements appear to be present even in the high-temperature AgNbO 3 polymorphs where the Nb cations, on average, reside on the ideal fixedcoordinate sites. The onset of the ͑a − b − c − ͒ / ͑a − b − c + ͒ tilting in the M polymorphs lifts the symmetry restrictions on the Nb positions and promotes ordering of the local Nb displacements into a long-range antipolarlike array. This ordering preserves the average Pbcm symmetry but is manifested in electron diffuse scattering and corroborated by other local-structure sensitive techniques. Structural states previously identified as the M 3 and M 2 phases represent different stages of displacive ordering rather than distinct thermodynamic phases. Rietveld refinements indicated intimate coupling between the displacive behavior on the oxygen, Nb, and Ag sublattices. The Pbcm symmetry of the octahedral framework precludes a complete ordering of Nb displacements so that some positional disorder is retained. This partial disorder likely gives a source to the dielectric relaxation which, according to previous spectroscopic studies, is the origin of the diffuse dielectric response exhibited by M-type AgNbO 3 at Ϸ250°C.

The room-temperature structure of Na ½ Bi ½ TiO 3 (NBT) ceramics was studied using several transmission electron microscopy (TEM) techniques. Highangle annular dark fi eld imaging in a scanning TEM confi rmed an essentially random distribution of Bi and Na, while electron diffraction revealed significant disorder of the octahedral rotations and cation displacements. Diffraction-contrast dark-fi eld and Fourier-fi ltered high-resolution TEM images were used to develop a model that reconciles local and average octahedral tilting in NBT. According to this model, NBT consists of nanoscale twin domains which exhibit a − a − c + tilting. The coherence length of the in-phase tilting, however, is limited to a few unit cells and is at least one order of magnitude shorter than that of anti-phase tilting. Assemblages of such nanodomains are proposed to exhibit an average a − a − c − tilt system. Diffuse sheets of intensity in electron diffraction patterns are attributed to local cation displacements correlated along both 〈 111 〉 and 〈 100 〉 chains and suggest partial polar ordering of these displacements. Overall, the TEM data indicate signifi cant chemical, cation-displacement and tilt disorder of the NBT structure at the nano and mesoscale and support the premise that the Cc symmetry recently proposed from powder diffraction refi nements is an averaged "best fi t" cell.

Tetragonal tungsten bronzes (TTBs), an important class of oxides known to exhibit ferroelectricity, undergo complex distortions, including rotations of oxygen octahedra, which give rise to either incommensurately or commensurately modulated superstructures. Many TTBs display broad, frequency-dependent relaxor dielectric behavior rather than sharper frequency-independent normal ferroelectric anomalies, but the exact reasons that favor a particular type of dielectric response for a given composition remain unclear. In this contribution the influence of incommensurate/commensurate displacive modulations on the onset of relaxor/ferroelectric behavior in TTBs is assessed in the context of basic crystal-chemical factors, such as positional disorder, ionic radii and polarizabilities, and point defects. We present a predictive crystal-chemical model that rationalizes composition–structure–properties relations for a broad range of TTB systems.

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 Ba 2 La x Nd 1−x Nb 3 Ti 2 O 15. 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 Ͼ 0.75, which remain incommensurate and exhibit only relaxor behavior below room temperature.