230 research outputs found
Some reduced ternary and quaternary oxides of molybdenum containing strong metal-metal bonds
Several new, reduced ternary and quaternary oxides of molybdenum are reported, each containing molybdenum in an average oxidation state \u3c 4.0. All of these compounds contain either discrete molybdenum atom clusters or infinite chains of bonded molybdenum atoms;The compounds ScZnMo(,3)O(,8), LiZn(,2)Mo(,3)O(,8), and Zn(,3)Mo(,3)O(,8) have been synthesized and crystal structures have been determined for the latter two. These oxides contain the same type of triangular molybdenum atom clusters found in the compound Zn(,2)Mo(,3)O(,8) (McCarroll, W. H. ; Katz, L; Ward, J. J. Am. Chem. Soc. 1957, 79, 5410). However, each of the trimeric clusters in these new compounds has available one or two additional electrons for participation in metal-metal bonding;Another newly prepared and characterized ternary oxide containing discrete metal atom clusters is Ba(,1.14)Mo(,8)O(,16). The structure of this compound consists of molybdenum-oxide cluster chains extended parallel with the c axis. These chains are built from clusters of the type Mo(,4)O(,16) sharing the oxygen atoms on the four outer edges of the planar tetrameric molybdenum atom cluster to give an Mo(,4)O(,8) stoichiometry. Two different infinite chains, built up from Mo(,4)O(,8)(\u272-) and Mo(,4)O(,8)(\u270.28-) cluster units, respectively, are interlinked via Mo-O-Mo bridge bonding to create four-sided tunnels in which the Ba(\u272+) ions reside;The new compound NaMo(,4)O(,6) contains infinite chains of bonded molybdenum atom clusters. These chains are comprised of clusters of the type Mo(,6)O(,12) fused at opposite edges by removal of two edge-bridging oxygen atoms, and sharing of the metal and remaining oxygen atoms between cluster units. The sodium ions occupy sites in channels formed by four molybdenum-oxide cluster chains crosslinked by strong Mo-O-Mo bonds;Another new compound, whose structure is closely related to that of NaMo(,4)O(,6), is Ba(,0.62)Mo(,4)O(,6). This material also exhibits a superlattice ordering of barium ions within the channels. An analysis of this superstructure from single crystal x-ray diffraction data is discussed;Other compounds that have been prepared and also partiallycharacterized by chemical analyses and x-ray powder diffractiondata are tentatively formulated as K(,2+x)Mo(,12)O(,19), Na(,2+x)MO(,12)O(,19),and CaMo(,5)O(,8);(\u271)DOE Report IS-T-960. This work was performed under Contract W-7405-eng-82 with the Department of Energy
Excitation Spectrum and Superexchange Pathways in the Spin Dimer VODPO_4 . 1/2 D_2O
Magnetic excitations have been investigated in the spin dimer material
VODPO_4 \cdot 1/2 D_2O using inelastic neutron scattering. A dispersionless
magnetic mode was observed at an energy of 7.81(4) meV. The wavevector
dependence of the scattering intensityfrom this mode is consistent with the
excitation of isolated V^{4+} spin dimers with a V-V separation of 4.43(7) \AA.
This result is unexpected since the V-V pair previously thought to constitute
themagnetic dimer has a separation of 3.09 \AA. We identify an alternative V-V
pair as the likely magnetic dimer, which involves superexchange pathways
through a covalently bonded PO_4 group. This surprising result casts doubt on
the interpretation of (VO)_2P_2O_7 as a spin ladder.Comment: 4 pages, 4 postscript figures - identical to previous paper but
figure 2 and 3 hopefully more compatible .p
Encapsulated Single Crystal Growth and Annealing of the High-Temperature Superconductor Tl-2201
Highly-perfect platelet single crystals of Tl_2Ba_2CuO_{6+d} (Tl-2201) were
grown by a self-flux technique. A novel encapsulation scheme allowed the
precursors to react prior to the sealing required to contain volatile thallium
oxides, and permitted the removal of melt at the conclusion of growth,
reproducibly producing high yields of clean crystals. The crystals were
annealed under well-controlled oxygen partial pressures, then characterised.
They have sharp superconducting transitions, narrow X-ray rocking curves and a
low 4% substitution of thallium by copper, all evidence of their high
perfection and homogeneity. The crystals are orthorhombic at most dopings, and
a previously unreported commensurate superlattice distortion is observed.Comment: 8 pages, 5 figures, submitted to Journal of Crystal Growt
Redetermination of Zn2Mo3O8
The crystal structure of dizinc trimolybdenum(IV) octaoxide, Zn2Mo3O8, has been redetermined from single-crystal X-ray data. The structure has been reported previously based on neutron powder diffraction data [Hibble et al. (1999 ▶). Acta Cryst. B55, 683-697] and single-crystal data [McCarroll et al. (1957 ▶). J. Am. Chem. Soc.
79, 5410–5414; Ansell & Katz (1966 ▶) Acta Cryst.
21, 482–485]. The results of the current redetermination show an improvement in the precision of the structural and geometric parameters with all atoms refined with anisotropic displacement parameters. The crystal structure consists of distorted hexagonal-close-packed oxygen layers with stacking sequence abac along [001] and is held together by alternating zinc and molybdenum layers. The Zn atoms occupy both tetrahedral and octahedral interstices with a ratio of 1:1. The Mo atoms occupy octahedral sites and form strongly bonded triangular clusters involving three MoO6 octahedra that are each shared along two edges, forming a Mo3O13 unit. All atoms lie on special positions. The Zn atoms are in 2b Wyckoff positions with 3m. site symmetry, the Mo atoms are in 6c Wyckoff positions with . m. site symmetry and the O atoms are in 2a, 2b and 6c Wyckoff positions with 3m. and . m. site symmetries, respectively
Band-structure trend in hole-doped cuprates and correlation with Tcmax
By calculation and analysis of the bare conduction bands in a large number of
hole-doped high-temperature superconductors, we have identified the energy of
the so-called axial-orbital as the essential, material-dependent parameter. It
is uniquely related to the range of the intra-layer hopping. It controls the Cu
4s-character, influences the perpendicular hopping, and correlates with the
observed Tc at optimal doping. We explain its dependence on chemical
composition and structure, and present a generic tight-binding model.Comment: 5 pages, Latex, 5 eps figure
Determination of incommensurate modulated structure in Bi2Sr1.6La0.4CuO6+{\delta} by aberration-corrected transmission electron microscopy
Incommensurate modulated structure (IMS) in Bi2Sr1.6La0.4CuO6+{\delta}
(BSLCO) has been studied by aberration corrected transmission electron
microscopy in combination with high-dimensional (HD) space description. Two
images in the negative Cs imaging (NCSI) and passive Cs imaging (PCSI) modes
were deconvoluted, respectively. Similar results as to IMS have been obtained
from two corresponding projected potential maps (PPMs), but meanwhile the size
of dots representing atoms in the NCSI PPM is found to be smaller than that in
PCSI one. Considering that size is one of influencing factors of precision,
modulation functions for all unoverlapped atoms in BSLCO were determined based
on the PPM obtained from the NCSI image in combination with HD space
description
C-axis lattice dynamics in Bi-based cuprate superconductors
We present results of a systematic study of the c axis lattice dynamics in
single layer Bi2Sr2CuO6 (Bi2201), bilayer Bi2Sr2CaCu2O8 (Bi2212) and trilayer
Bi2Sr2Ca2Cu3O10 (Bi2223) cuprate superconductors. Our study is based on both
experimental data obtained by spectral ellipsometry on single crystals and
theoretical calculations. The calculations are carried out within the framework
of a classical shell model, which includes long-range Coulomb interactions and
short-range interactions of the Buckingham form in a system of polarizable
ions. Using the same set of the shell model parameters for Bi2201, Bi2212 and
Bi2223, we calculate the frequencies of the Brillouin-zone center phonon modes
of A2u symmetry and suggest the phonon mode eigenvector patterns. We achieve
good agreement between the calculated A2u eigenfrequencies and the experimental
values of the c axis TO phonon frequencies which allows us to make a reliable
phonon mode assignment for all three Bi-based cuprate superconductors. We also
present the results of our shell model calculations for the Gamma-point A1g
symmetry modes in Bi2201, Bi2212 and Bi2223 and suggest an assignment that is
based on the published experimental Raman spectra. The
superconductivity-induced phonon anomalies recently observed in the c axis
infrared and resonant Raman scattering spectra in trilayer Bi2223 are
consistently explained with the suggested assignment.Comment: 29 pages, 13 figure
Non-centrosymmetric Na3Nb4As3O19
A new non-centrosymmetric compound, trisodium tetraniobium triarsenic nonadecaoxide, Na3Nb4As3O19, has been synthesized by a solid-state reaction at 1123 K. The structure consists of AsO4 tetrahedra and NbO6 octahedra sharing corners to form a three-dimensional framework containing two types of tunnels running along the c axis, in which the sodium ions are located. Na+ cations occupying statistically several sites, respectively, are surrounded by seven, six and four O atoms at distances ranging from 2.08 (1) to 2.88 (4) Å. The title structure is compared with those containing the same groups, viz.
M
2XO13 and M
2
X
2O17 (M = transition metal, and X = As or P)
Correlated local distortions of the TlO layers in TlBaCuO: An x-ray absorption study
We have used the XAFS (x-ray-absorption fine structure) technique to
investigate the local structure about the Cu, Ba, and Tl atoms in orthorhombic
Tl-2201 with a superconducting transition temperature T=60 K. Our results
clearly show that the O(1), O(2), Cu, and Ba atoms are at their ideal sites as
given by the diffraction measurements, while the Tl and O(3) atoms are more
disordered than suggested by the average crystal structure. The Tl-Tl distance
at 3.5 \AA{ } between the TlO layers does not change, but the Tl-Tl distance at
3.9 \AA{ } within the TlO layer is not observed and the Tl-Ba and Ba-Tl peaks
are very broad. The shorter Tl-O(3) distance in the TlO layer is about 2.33
\AA, significantly shorter than the distance calculated with both the Tl and
O(3) atoms at their ideal sites ( 0 or ). A model based
on these results shows that the Tl atom is displaced along the
directions from its ideal site by about 0.11 \AA; the displacements of
neighboring Tl atoms are correlated. The O(3) atom is shifted from the $4e$
site by about 0.53 \AA{ } roughly along the directions. A comparison of
the Tl L-edge XAFS spectra from three samples, with T=60 K, 76 K,
and 89 K, shows that the O environment around the Tl atom is sensitive to T
while the Tl local displacement is insensitive to T and the structural
symmetry. These conclusions are compared with other experimental results and
the implications for charge transfer and superconductivity are discussed. This
paper has been submitted to Phys. Rev. B.Comment: 20 pages plus 14 ps figures, REVTEX 3.
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