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A facility for X-ray diffraction has been developed at the National High Magnetic Field Laboratory. It brings diffraction capability to the 25 T Florida split coil magnet and implements temperature control in a range of 15-295 K using a cold finger helium cryostat. This instrument represents an alternative to pulsed magnetic field systems, and it exceeds the static magnetic fields currently available at synchrotron facilities. Magnetic field compatibility of an X-ray source and detectors with the sizable magnetic fringe fields emanating from the magnet constrained the design of the diffractometer. [ABSTRACT FROM AUTHOR]

ZnO nanotips are grown on epitaxial GaN/c-sapphire templates by metalorganic chemical vapor deposition. X-ray diffraction (XRD) studies indicate that the epitaxial relationship between ZnO nanotips and the GaN layer is (0002)ZnO∥(0002)GaN and (10̄10)ZnO∥(10̄10)GaN. Temperature-dependent photoluminescence (PL) spectra have been measured. Sharp free exciton and donor-bound exciton peaks are observed at 4.4 K with photon energies of 3.380 eV, 3.369 eV, and 3.364 eV, confirming high optical quality of ZnO nanotips. Free exciton emission dominates at temperatures above 50 K. The thermal dissociation of these bound excitons forms free excitons and neutral donors. The thermal activation energies of the bound excitons at 3.369 eV and 3.364 eV are 11 meV and 16 meV, respectively. Temperature-dependent free A exciton peak emission is fitted to the Varshni's equation to study the variation of energy bandgap versus temperature. [ABSTRACT FROM AUTHOR]

Single-crystal X-ray diffraction studies indicate that the compound Ba27Fe16Ti33O117 crystallizes in the rhombohedral space group R3m, with a hexagonal unit cell a=5.7400(8), c=127.11(3) A˚; Z=1.5. The arrangement may be described as a 54-layer (54L) close-packed structure (stacking sequence (cchcchhhhcchhhhcch)3) built from oxygen and {Ba,O} layers, with Ti4+ occupying octahedra and Fe3+ occupying both octahedral and tetrahedral interstices. The 54L structure contains hexagonal 6L BaTiO3-type (cch)2 units via a 9-fold repeat of the 6L stacking sequence, with iron preferentially occupying layers centered around z=1/6. Ba27Fe16Ti33O117 melts incongruently at 1270 °C and is difficult to purify in polycrystalline form, although crystals are easily obtained from partial melts. The new compound is a member of a family of ternary Ba&z.sbnd;Fe&z.sbnd;Ti&z.sbnd;O phases that may be considered as dielectric–magnetic hybrids of barium-polytitanate and barium-hexaferrite crystal chemistries. [ABSTRACT FROM AUTHOR]

Nonpolar a-plane (1120) MgxZn1-xO (a-MgxZn1-xO) films are deposited on (0112) r-sapphire substrates using metalorganic chemical vapor deposition with varying Mg composition (x from 0 to 0.25). Unit cell parameters with Mg composition are determined by high-resolution triple-axis x-ray diffraction. In-plane strain along the c-axis [0001] and m-axis [1100] in the films is anisotropic and increases with increasing Mg composition. The in-plane strain anisotropy changes with Mg composition in a-MgxZn1-xO. Calculations are carried out to determine the influence of Mg content on the residual interfacial strain. [ABSTRACT FROM AUTHOR]

We deposited [formula] nonpolar a-plane ZnO (a-ZnO) films on (0112) r-sapphire substrates using metalorganic chemical vapor deposition. Unit cell deformation due to interfacial strain in films was determined by triple-axis x-ray diffraction. Due to low symmetry of a-plane, anisotropic strain is observed along the [0001] (c-axis) and [1100] (m-axis) in-plane axes. Out-of-plane strain along [1120] (a-axis) is tensile and relaxes for film thickness >=2 μm. The in-plane m-axis is under tensile strain and c-axis is under compressive strain. Increase in film thickness increases in-plane strain anisotropy due to faster relaxation along the m-axis. Thermal and lattice mismatch strains are separated by curve fitting. [ABSTRACT FROM AUTHOR]

Fe ions of dose 5×1016 cm-2 were implanted at 200 keV into a-plane ZnO epitaxial films. The epitaxial quality of the postannealed samples was verified by x-ray diffraction ω-rocking curves and φ scans, whereas x-ray absorption spectroscopy identified the presence of both Fe2+ and Fe3+ ions, as well as changes in their relative concentration during postannealing. Superconducting quantum interference device measurements show that the as-implanted and postannealed films are ferromagnetic at room temperature. The saturation magnetization reduces during annealing possibly due to the decrease in the number of oxygen vacancies. [ABSTRACT FROM AUTHOR]

The structure of the mixed-valent Ba5Fe5O14 (BaFeO2.8), prepared using a molten KOH–Ba(OH)2 flux, has been determined using single crystal X-ray diffraction. Ba5Fe5O14 forms twinned crystals with the orthorhombic space group Cmcm, a=5.7615(8), b=9.9792(14) and c=24.347(3) A˚, Z=4. The structure, which is closely related to the 10H BaFeO2.65 perovskite, is composed of two oxygen-deficient BaO2 layers and eight BaO3 layers with a (hchhc)2 stacking sequence, where h and c denote hexagonal or cubic layers. A displacement of barium and oxygen atoms in the BaO2 layers from hexagonal special positions lowers the symmetry from hexagonal to orthorhombic. This combination of stacking and vacancies creates trimers of face-sharing FeO6 octahedra pillared by dimers of corner-sharing FeO4 tetrahedra. The Fe4+ atoms are located in the center of the trimer and in the tetrahedral sites. The magnetism of Ba5Fe5O14, investigated using SQUID magnetometry, is characteristic of a strongly coupled antiferromagnet. [Copyright &y& Elsevier]

Subsolidus phase relations have been determined for the BaO:TiO2:Ta2O5 system by X-ray diffraction analysis of ∼100 specimens prepared in air at temperatures near but below the solidus (1275 °C to 1500 °C). For the BaO:Ta2O5 subsystem, the formation of all three polymorphs of BaTa2O6 (orthorhombic, tetragonal, and hexagonal) was confirmed. The present study found no stable phases forming between Ba5Ta4O15 and BaO. In the ternary system, along the line BaTiO3–Ba5Ta4O15, the previously reported 8L and 10L hexagonal perovskite derivatives were confirmed, albeit with somewhat wider homogeneity ranges; BaTiO3 dissolves Ta5+ up to BaTi0.64Ta0.29O3, which contains 7% B-cation vacancies and exhibits a dilated cubic unit cell (a=4.0505(2) A˚). The formation of Ba3Ti4Ta4O21, a member of the hexagonal A3M8O21-type ternary oxides, was confirmed as well as its solid solution, for which a different mechanism of formation is suggested. Several new compounds have been found, including four members of the orthorhombic (space group Cmcm) “rutile-slab” homologous series, BaTinTa4O11+2n, with n-values 3,5,7,9; the unit cells for these compounds are given. Three ternary phases with close-packed [Ba,O] layer structures related to that of 6L Ba4Ti13O30 were found: 13L Ba18Ti53Ta2O129, 7L Ba10Ti27Ta2O69, and 8L Ba6Ti14Ta2O39. The crystal structures of the 13L and 7L phases were determined by single-crystal X-ray diffraction (13L Ba18Ti53Ta2O129: cchcchcchcchc, C2/m, a=9.859(2) A˚, b=17.067(5) A˚, c=30.618(8) A˚, β=96.11(2)°, Z=2; 7L Ba10Ti27Ta2O69: cchcchc, C2/m, a=9.855(3) A˚, b=17.081(7) A˚, c=16.719(7) A˚, β=101.18(3)°; Z=2), and are described and compared with those of the 6L and 8L members of this family. Phases with tetragonal tungsten bronze (TTB) related structures occur over large compositional ranges in the BaO:TiO2:Ta2O5 system, both within the ternary (Ba6−xTi2−2xTa8+2xO30, x=0→0.723), and along the BaO–Ta2O5 binary (from 43 to 26 mol% BaO), which in turn dissolves up to ∼12 mol% TiO2 to form extensive single-phase fields with different TTB superstructures. X-ray powder diffraction data are given for three binary BaO:Ta2O5 compounds exhibiting the TTB structure and two superstructures. The large compositional ranges of the TTB-type phases are accompanied by only small changes in specific volume. The TTB-type regions within the ternary and those emanating from the BaO–Ta2O5 binary do not co-exist in equilibrium, which likely reflects fundamental differences in the structural mechanisms that accommodate the variable stoichiometries. The similarities and differences between the BaO:TiO2:Ta2O5 and BaO:TiO2:Nb2O5 systems are described. [Copyright &y& Elsevier]

Single crystals of LnMg 2 Bi 2 (Ln=Yb, Eu, Sm) and SmMg 2 Sb 2 were synthesized using Mg–Bi metal and Mg–Sb metal fluxes, respectively. The crystal structures are of the CaAl 2 Si 2 type with space group P 3 ̅ m1 (#164, Z =1): SmMg 2 Bi 2 ( a =4.7745(1) Å, c =7.8490(2) Å), EuMg 2 Bi 2 ( a =4.7702(1) Å, c =7.8457(2) Å), YbMg 2 Bi 2 ( a =4.7317(2) Å, c =7.6524(3) Å), and SmMg 2 Sb 2 ( a =4.6861(1) Å, c =7.7192(2) Å). Heat capacity, electrical transport, and magnetization of all bismuth containing phases were measured. The materials behave as “poor metals” with resistivity between 2 and 10 mΩ cm. Temperature independent Van Vleck paramagnetism is observed in SmMg 2 Bi 2 indicative of divalent samarium (Sm 2+ ) ions. [ABSTRACT FROM AUTHOR]

Single-crystal x-ray diffraction, magnetic susceptibility, magnetization, heat capacity, and electrical resistivity measurements are reported for the new tetragonal compound Ce2PdGe3, which forms in the space group P42/mmc (No. 131)--a relative of the a-ThSi2-type structure. Measurements reveal a two-part antiferromagnetic phase transition at Tn,1 = 10.7 K and TN,2 = 9.6 K and subsequent ferromagnetlike ordering near Tc = 2.25 K. The ordered ground state emerges from a lattice of Ce ions that are hybridized with the conduction electrons, as revealed by the enhanced electronic coefficient of the specific heat γ ≈ 50 mJ/mol-Ce-K2 (extrapolated to T = 0 for T < Tc ). Electronic structure calculations reveal a Fermi surface that includes sheets with distinct nesting vectors. Chemical/structural disorder also plays an important role, as evidenced by results from single-crystal x-ray diffraction, the width of the peaks in the heat capacity at 7k and Tc, and the small residual resistivity ratio RRR = ρ300K/ρ0 = 1-8. [ABSTRACT FROM AUTHOR]

Abstract: Single crystals of the spin dimer compound Pb2V3O9 were grown for the first time by the floating zone method. Room temperature single-crystal X-ray diffraction measurements confirm their crystallinity and structure. The crystal growth process is discussed in detail and high temperature differential scanning calorimetry measurements that show a structural phase transition from a monoclinic to a triclinic unit cell at 759(1)K are presented. [Copyright &y& Elsevier]