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Budai, John D

Publications and source records attributed to Budai, John D.

6 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Nanoscale structure in AgSbTe2 determined by diffuse elastic neutron scattering

Diffuse elastic neutron scattering measurements confirm that AgSbTe2 has a hierarchical structure, with defects on length scales from nanometers to microns. While scattering from mesoscale structure is consistent with previously-proposed structures in which Ag and Sb order on a NaCl lattice, more diffuse scattering from nanoscale structure suggests a structural rearrangement in which hexagonal layers form a combination of (ABC), (ABA), and (AAB) stacking sequences. The AgCrSe2 structure is the best-fitting model for the local atomic arrangements.

Specht, Eliot D↗
DOE OSTI2014

Focused-ion-beam induced damage in thin films of complex oxide BiFeO{sub 3}

An unexpected, strong deterioration of crystal quality is observed in epitaxial perovskite BiFeO{sub 3} films in which microscale features have been patterned by focused-ion-beam (FIB) milling. Specifically, synchrotron x-ray microdiffraction shows that the damaged region extends to tens of μm, but does not result in measureable changes to morphology or stoichiometry. Therefore, this change would go undetected with standard laboratory equipment, but can significantly influence local material properties and must be taken into account when using a FIB to manufacture nanostructures. The damage is significantly reduced when a thin metallic layer is present on top of the film during the milling process, clearly indicating that the reduced crystallinity is caused by ion beam induced charging.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Phonon localization drives polar nanoregions in a relaxor ferroelectric

Relaxor ferroelectrics1, which are utilized as actuators and sensors2-4, exemplify a class of poorly understood materials where interplay between disorder and phase instability results in inhomogeneous nanoregions. There is no definitive explanation for the onset of relaxor behavior (Burns temperature5, Td) or the origin of polar nanoregions (PNRs). Here we show a vibrational mode that localizes on cooling to Td, remains localized as PNRs form, and then delocalizes as PNRs grow using neutron scattering on relaxor (Pb(Mg1/3Nb2/3)O3)0.69-(PbTiO3)0.31 (PMN-31%PT). Although initially appearing like intrinsic local modes (ILMs)6-10, these modes differ below Td as they form a resonance with the ferroelectric phonon. At the resonance, nanoregions of standing ferroelectric phonons develop with a coherence length matching the PNRs. The size, shape, distribution, and temporal fluctuations of PNRs, and our observations, are explained by ferroelectric phonons trapped by disordered resonance modes via Anderson localization11-13. Our results show the size and shape of PNRs are not dictated by complex structural details, as always assumed, but by a phonon resonance wavevector. This simplification could guide the design of next generation relaxors.

Manley, Michael E↗
DOE OSTI2014

Neutron Scattering Investigation of Phonon Scattering Rates in Ag1-xSb1+xTe2+x (x = 0, 0.1, and 0.2)

The phonon dispersions and scattering rates of the thermoelectric material AgSbTe$_{2}$ were measured as a function of temperature with inelastic neutron scattering. The results show that phonon scattering rates are large and weakly dependent on temperature. The lattice thermal conductivity was calculated from the measured phonon lifetimes and group velocities, providing good agreement with bulk transport measurements. The measured phonon scattering rates and their temperature dependence are compared with models of phonon scattering by anharmonicity and point defect. We find that these processes cannot account for the large total phonon scattering rates observed, and their lack of temperature dependence. Neutron and synchrotron diffraction measurements on single crystals revealed an extensive nanostructure from cation ordering, which is likely responsible for the strong phonon scattering.

Abernathy, Douglas L↗
DOE OSTI2014

EMERGING CAPABILITIES FOR MATERIALS CHARACTERIZATION WITH POLYCHROMATIC MICRODIFFRACTION8

Polychromatic microdiffraction is an emerging tool for mapping local crystal structure with submicron three-dimensional resolution. The method is sensitive to the local crystal phase, crystallographic orientation, elastic strain, and lattice curvature. For many materials it is also nondestructive, which allows for unique experiments that probe how particular structural configurations evolve during processing and service. This capability opens up the possibility of testing and guiding theories without the limitations imposed by destructive techniques, surface-limited measurements or ensemble averages. This new capability will impact long-standing issues of materials science ranging from the factors that control anisotropic materials deformation to factors that influence grain growth, grain boundary migration, electromigration and stress driven materials evolution. Such mesoscopic phenomena are at the heart of virtually all materials processing and form the basis for modern materials engineering. Here we describe the state-of-the-art, and discuss new instrumentation with the promise of better sensitivity and better real and reciprocal space resolution. Example science and future research opportunities are described.

Ice, Gene E↗