In many applications involving functional oxides, composite structures consisting of multiple oxides and interfaces between the two are of particular interest, as they provide enhanced properties over the individual phases. Often, materials intended for radioisotope immobilization are composite structures, consisting of multiple chemistries and structures. In this work, two pyrochlore materials, Gd _2 Zr _2 O _7 (GZO) and Gd _2 Ti _2 O _7 (GTO) are interfaced in a bilayer structure and irradiated to test the composite’s capacity to accommodate lattice point defects and the potential for cation transport across the interface. Using x-ray energy dispersive spectroscopy after the pristine bilayer was irradiated to damage levels of 0.2–0.8 dpa using a 12 MeV Cu ^4+ ion beam, significant cation intermixing was observed by the highest dose. While, as might be expected, the bulk of the GTO layer easily amorphized, surprisingly, the GZO layer also amorphized with arelatively small dose. More interestingly, the structure maintained a crystalline layer at the original interface between the two pyrochlores, even though the interface moved significantly during the irradiation. These results are explained through a physical model for ballistic mixing in pyrochlore. These results highlight the complex structural response of oxide heterostructures under extreme conditions.

A series of transition metal NixCo1-x(N(CN)(2))(2) dicyanamide molecular magnet, 0 <= x <= 1, was chemically synthesized. Structure and magnetic properties were investigated by IR spectroscopy, neutron powder diffraction and temperature-dependent magnetization measurement. The neutron powder diffraction measurement established that all samples showed the rutile structure with Pnnm space group. The detailed Rietveld refinement results revealed a systematic decrease of unit-cell volume with increasing Ni content. We also observed a concurrent increase of the Curie temperature with Ni content due to the increase of superexchange interaction. (C) 2016 Elsevier B.V. All rights reserved.

We report transport measurement in zero and applied magnetic field on a single crystal of NbAs. Transverse and longitudinal magnetoresistance in the plane of this tetragonal structure does not saturate up to 9 T. In the transverse configuration (H ∥ c, I ⊥ c) it is 230,000% at 2 K. The Hall coefficient changes sign from hole-like at room temperature to electron-like below ∼150 K. The electron carrier density and mobility calculated at 2 K based on a single band approximation are 1.8 × 10(19) cm(-3) and 3.5 × 10(5) cm(2) Vs(-1), respectively. These values are similar to reported values for TaAs and NbP, and further emphasize that this class of noncentrosymmetric, transition-metal monopnictides is a promising family to explore the properties of Weyl semimetals and the consequences of their novel electronic structure.

The synthesis, crystal structure, and physical properties studied by means of x-ray diffraction, magnetic, thermal and transport measurements of CeMAl_4Si_2 (M = Rh, Ir, Pt) are reported, along with the electronic structure calculations for LaMAl_4Si_2 (M = Rh, Ir, Pt). These materials adopt a tetragonal crystal structure (space group P4/mmm) comprised of BaAl_4 blocks, separated by MAl_2 units, stacked along the c-axis. Both CeRhAl_4Si_2 and CeIrAl_4Si_2 order antiferromagnetically below T_N1=14 and 16 K, respectively, and undergo a second antiferromagnetic transitition at lower temperature (T_N2=9 and 14 K, respectively). CePtAl_4Si_2 orders ferromagnetically below T_C =3 K with an ordered moment of μ_sat=0.8 μ_B for a magnetic field applied perpendicular to the c-axis. Electronic structure calculations reveal quasi-2D character of the Fermi surface.

We report magnetization M, heat capacity C, and electrical resistivity p for single crystals of the itinerant electron antiferromagnet CaCo2P2 (T-N approximate to 110 K). Measurements at ambient pressure reveal rich magnetic behavior, where ferromagnetic correlations are present in the paramagnetic state and a subsequent feature is seen at T-1 approximate to 22 K within the ordered state. Heat-capacity measurements additionally reveal moderately enhanced electronic correlations, as evidenced by the electronic coefficient of the specific heat gamma = 23 mJ/mol center dot K-2, which is large by comparison to closely related 122 analogs and the value predicted by electronic structure calculations. Upon the application of pressure, T-N is suppressed toward zero. For P >= 0.89 GPa, another phase transition appears at T-2 < T-N which is also suppressed by P. At P-c approximate to 1.4-1.5 GPa, T-N and T-2 drop abruptly to zero at a putative quantum phase transition. ForP > P-c, a broad shoulder in rho(T) appears at T*, which moves to higher T and broadens with increasing P. We discuss possible scenarios to understand the phase diagram and compare to other compounds which show similar P-driven behavior.

Single crystals of (Ca1-xLax)10(Pt3As8)(Fe2As2)5 (x = 0 to 0.182) superconductors have been grown and characterized by X-ray, microprobe, transport and thermodynamic measurements. Features in the magnetic susceptibility, specific heat and two kinks in the derivative of the electrical resistivity around 100 K in the x = 0 compound support the existence of decoupled structural and magnetic phase transitions. With La doping, the structural/magnetic phase transitions are suppressed and a half-dome of superconductivity with a maximal Tc around 26 K is observed in the temperature-concentration phase diagram.

Ga{sub 2}S{sub 3}-Na{sub 2}S (GNS) glasses doped with CsCl were synthesized in open crucibles under inert atmosphere. The evaporative loss of CsCl during glass melting was measured by energy dispersive X-ray spectroscopy and corrected for by biasing the CsCl concentration in the mixture of starting materials to obtain glasses with accurately controlled stoichiometry. Glass transition temperatures, refractive index dispersions, and band edge energies were measured for four GNS:CsCl glasses, and the respective values were found to significantly improve over earlier studies that did not mitigate CsCl evaporative losses. The refractive index dispersion measurements indicate that the Cs{sup +} and Cl{sup -} radii are 16% larger in GNS:CsCl glass than in bulk crystalline CsCl. The band edge energy increases from 2.97 eV in GNS glass to 3.32 eV in GNS glass doped with 20 mol% CsCl as a result of introducing Cl{sup -} ions having a large optical electronegativity. The large bandgap of 3.32 eV and the low (450 cm{sup -1}) phonon energy make GNS:20%CsCl an attractive host material for rare-earth ions with radiative transitions in the near ultra-violet, visible, and near-infrared spectral regions.

A possible path for fabricating three-dimensional metamaterials with curved geometries at optical and infrared frequencies is to stack flexible metamaterial layers. We have fabricated highly uniform metamaterials at terahertz frequencies on large-area, low-stress, free-standing 1 μm thick silicon nitride membranes. Their response remains comparable to that of similar structures on thick substrates as measured by the quality factor of the resonances. Transmission measurements with a Fourier transform infrared spectrometer highlight the advantage of fabricating high frequency metamaterials on thin membranes as etalon effects are eliminated. Releasing the membranes enables layering schemes and placement onto curved surfaces in order to create three-dimensional structures.

We present terahertz metamaterials fabricated on large-area, free-standing thin (les1 mum) silicon nitride membranes with the aim of reducing dielectric losses, enhancing metamaterial sensing capabilities, and enabling flexible and conformable designs.

The terahertz (THz) region of the electromagnetic spectrum holds promise for spectroscopic imaging of illicit and hazardous materials, and chemical fingerprinting using moment of inertia vibrational transitions. Passive and active devices operating at THz frequencies are currently a challenge, and a promising emerging technology for such devices is optical metamaterials. In particular, a chem/bio sensing scheme based on the sensitivity of metamaterials to their dielectric environment has been proposed but may be limited due to the large concentration of electric flux in the substrate. In addition, there is an interest in fabricating 3D metamaterials, which is a challenge at these and shorter wavelengths due to fabrication constraints. In order to address both of these problems, we have developed a process to fabricate THz metamaterials on free-standing, 1 micron thick silicon nitride membranes. We will present THz transmission spectra and the corresponding simulation results for these metamaterials, comparing their performance with previously fabricated metamaterials on various thick substrates. Finally, we will present a scheme for implementing a 3D THz metamaterial based on stacking and possibly liftoff of these silicon nitride membranes.

The crystal structure of one of the simplest organoboron compounds, trimethyl borate does not appear to have been determined hitherto. The compound is of interest for the study of pi-donor ligands and their interaction with the pi-acceptor behavior of trigonal boron and the consequences of such interactions on molecular structure. We used powder neutron (with isotopically labeled material) and X-ray diffraction to determine the crystal structure of trimethyl borate at 15 K and 200 K (neutron) and 200 K (X-ray). The material is hexagonal (Z = 2) with a b = 6.950(8) angstrom and c = 6.501(3) angstrom at 15 K. The unit cell volume is 272.00(1) angstrom(3). The space group is P6(3)/M (SG 176) at 15 K and 200 K. This is the first crystal structure solved on the Neutron Powder Diffractometer (NPDF) at the Lujan Center.

The LIGA microfabrication technique offers a unique method for fabricating 3-dimensional photonic lattices based on the Iowa State "logpile" structure. These structures represent the [111] orientation of the [100] logpile structures previously demonstrated by Sandia National Laboratories. The novelty to this approach is the single step process that does not require any alignment. The mask and substrate are fixed to one another and exposed twice from different angles using a synchrotron light source. The first exposure patterns the resist at an angle of 45 degrees normal to the substrate with a rotation of 8 degrees. The second exposure requires a 180 degree rotation about the normal of the mask and substrate. The resulting pattern is a vertically oriented logpile pattern that is rotated slightly off axis. The exposed PMMA is developed in a single step to produce an inverse lattice structure. This mold is filled with electroplated gold and stripped away to create a usable gold photonic crystal. Tilted logpiles demonstrate band characteristics very similar to those observed from [100] logpiles. Reflectivity tests show a band edge around 5 μm and compare well with numerical simulations.

We have designed, fabricated, and tested large sheets of photonic bandgap (PBG) material that have a "cubic array of cubes" structure. Structures with bandgaps in two wavebands have been fabricated: the thermal IR (8-12 mu m) and the visible/near IR (0.6-2.5 mu m). A thermal-IR PBG can modify the emission properties of structures for temperature control. Visible/near-IR PBGs can be used in photonic circuits and can improve illumination efficiency.

Structural analysis by time-of-flight neutron diffraction is presented for both ErD2±x powders and film samples. Rietveld refinement results demonstrated sensitivity to deuterium site occupancy and distribution within the fluorite-type lattice. Powder refinements showed that the presence of 10% D at the octahedral site caused detectable contraction in the unit cell volume. XRD analysis revealed strong bi-axial texture in the ErD2 thin films. With a sufficient number of films present, neutron diffraction analysis and refinement of the ErD2 structure could be achieved.

We discuss the impact of strong absorption for thermal neutrons on data analysis and compare absorption corrections in the GSAS and MAUD Rietveld codes for texture and structural parameter refinement. Diffraction data were collected on the neutron powder diffractometer HIPPO at LANSCE from dysprosium and erbium, which are moderate-to-strong absorbers for thermal neutrons with absorption cross sections of 159 barns for Er and 994 barns for Dy at λ=1.8 Å. Both elements have hexagonal-close-packed (hcp) crystal structures, and the samples were various thicknesses of rolled foils. The orientation distribution functions (ODF) were fit to the same neutron time-of-flight data sets using two very different full pattern Rietveld analysis procedures. Spherical harmonics functions were fit to the textured data using GSAS. These data were also analyzed by the modified direct method E-WIMV using MAUD. The resulting pole figures from the ODFs determined by both Rietveld analysis packages are qualitatively similar, and the textures were confirmed by X-ray diffraction. Additionally, data from orthorhombic dysprosium and erbium fluoride powders show that atomic positions are not sensitive to absorption. We address inconsistencies and methodologies in data analysis when strong absorption is present.

The crystal structures, atomic distributions, and theoretical electronic structures of five different Cu5-xZn8+x gamma-brass compounds (x = -0.59(3), -0.31(3), 0.00(3), 0.44(3), and 0.79(3)) are reported with the goal of identifying chemical influences on the observed phase width. These structures have been refined by both neutron and X-ray powder diffraction to obtain accurate crystal chemical parameters. All compounds crystallize in the space group Iz3m (No. 217) (Z = 4), and the unit cell parameters are a = 8.8565(4), 8.8612(5), 8.8664(3), 8.8745(4), and 8.8829(7) A, respectively, for Cu5.59Zn7.41, Cu5.31Zn7.69, Cu5.00Zn8.00, Cu4.56Zn8.44, and Cu4.21Zn8.79. The results indicate specific site substitutions on both sides of the ideal composition "Cu5Zn8". In all cases, the 26-atom cluster building up the -brass structure shows a constant inner [Cu4Zn4] tetrahedral star with compositional variation occurring at the outer octahedron and cuboctahedron. First principles and semiempirical electronic structure calculations using both a COHP and Mulliken population analysis were performed to understand the observed compositional range and to address the "coloring problem" for the site preferences of Cu and Zn atoms for this series of compounds.