133 research outputs found
Ferromagnetism in Co7(TeO3)4Br6: A byproduct of complex antiferromagnetic order and single-ion anisotropy
Pronounced anisotropy of magnetic properties and complex magnetic order of a
new oxi-halide compound Co7(TeO3)4Br6 has been investigated by powder and
single crystal neutron diffraction, magnetization and ac susceptibility
techniques. Anisotropy of susceptibility extends far into the paramagnetic
temperature range. A principal source of anisotropy are anisotropic properties
of the involved octahedrally coordinated single Co(2+) ions, as confirmed by
angular-overlap-model calculations presented in this work. Incommensurate
antiferromagnetic order sets in at TN=34 K. Propagation vector is strongly
temperature dependent reaching k1=(0.9458(6), 0, 0.6026(5)) at 30 K. A
transition to a ferrimagnetic structure with k2=0 takes place at TC=27 K.
Magnetically ordered phase is characterized by very unusual anisotropy as well:
while M-H scans along b-axis reveals spectacularly rectangular but otherwise
standard ferromagnetic hysteresis loops, M-H studies along other two principal
axes are perfectly reversible, revealing very sharp spin flop (or spin flip)
transitions, like in a standard antiferromagnet (or metamagnet). Altogether,
the observed magnetic phenomenology is interpreted as an evidence of competing
magnetic interactions permeating the system, first of all of the single ion
anisotropy energy and the exchange interactions. Different coordinations of the
Co(2+)-ions involved in the low-symmetry C2/c structure of Co7(TeO3)4Br6 render
the exchange-interaction network very complex by itself. Temperature dependent
changes in the magnetic structure, together with an abrupt emergence of a
ferromagnetic component, are ascribed to continual spin reorientations
described by a multi-component, but yet unknown, spin Hamiltonian.Comment: 12 pages, 13 figures; submitted to PR
Magnetic properties of the geometrically frustrated S=1/2 antiferromagnets, La2LiMoO6 and Ba2YMoO6, with the B-site ordered double perovskite structure: Evidence for a Collective Spin Singlet Ground State
Two B-site ordered double perovskites, La2LiMoO6 and Ba2YMoO6, based on the S
= 1/2 ion, Mo5+, have been investigated in the context of geometric magnetic
frustration. Powder neutron diffraction, heat capacity, susceptibility, muon
spin relaxation(_SR), and 89Y NMR- including MAS NMR- data have been collected.
La2LiMoO6 deviates strongly from simple Curie-Weiss paramagnetic behavior below
150K and zero-field cooled/ field cooled (ZFC/FC)irreversibility occurs below
20K with a weak, broad susceptibility maximum near 5K in the ZFC data. A
Curie-Weiss fit shows a reduced mu_eff=1.42\mu_B, (spin only = 1.73 muB) and a
Weiss temperature, \theta_c, which depends strongly on the temperature range of
the fit. Powder neutron diffraction, heat capacity and 7Li NMR show no evidence
for long range magnetic order to 2K. On the other hand oscillations develop
below 20K in muSR indicating at least short range magnetic correlations.
Susceptibility data for Ba2YMoO6 also deviate strongly from the C-W law below
150K with a similarly reduced mu_eff = 1.72\mu_B and \theta_c = - 219(1)K. Heat
capacity, neutron powder diffraction and muSR data show no evidence for long
range order to 2K but a very broad maximum appears in the heat capacity. The
89Y NMR paramagnetic Knight shift shows a remarkable local spin susceptibility
behavior below about 70K with two components from roughly equal sample volumes,
one indicating a singlet state and the other a strongly fluctuating
paramagnetic state. Further evidence for a singlet state comes from the
behavior of the relaxation rate, 1/T1. These results are discussed and compared
with those from other isostructural S = 1/2 materials and those based on S =
3/2 and S = 1.Comment: Accepted for publication in Phys. Rev.
X-ray absorption spectroscopy systematics at the tungsten L-edge
A series of mononuclear six-coordinate tungsten compounds spanning formal oxidation states from 0 to +VI, largely in a ligand environment of inert chloride and/or phosphine, has been interrogated by tungsten L-edge X-ray absorption spectroscopy. The L-edge spectra of this compound set, comprised of [W<sup>0</sup>(PMe<sub>3</sub>)<sub>6</sub>], [W<sup>II</sup>Cl<sub>2</sub>(PMePh<sub>2</sub>)<sub>4</sub>], [W<sup>III</sup>Cl<sub>2</sub>(dppe)<sub>2</sub>][PF<sub>6</sub>] (dppe = 1,2-bis(diphenylphosphino)ethane), [W<sup>IV</sup>Cl<sub>4</sub>(PMePh<sub>2</sub>)<sub>2</sub>], [W<sup>V</sup>(NPh)Cl<sub>3</sub>(PMe<sub>3</sub>)<sub>2</sub>], and [W<sup>VI</sup>Cl<sub>6</sub>] correlate with formal oxidation state and have usefulness as references for the interpretation of the L-edge spectra of tungsten compounds with redox-active ligands and ambiguous electronic structure descriptions. The utility of these spectra arises from the combined correlation of the estimated branching ratio (EBR) of the L<sub>3,2</sub>-edges and the L<sub>1</sub> rising-edge energy with metal Z<sub>eff</sub>, thereby permitting an assessment of effective metal oxidation state. An application of these reference spectra is illustrated by their use as backdrop for the L-edge X-ray absorption spectra of [W<sup>IV</sup>(mdt)<sub>2</sub>(CO)<sub>2</sub>] and [W<sup>IV</sup>(mdt)<sub>2</sub>(CN)<sub>2</sub>]<sup>2–</sup> (mdt<sup>2–</sup> = 1,2-dimethylethene-1,2-dithiolate), which shows that both compounds are effectively W<sup>IV</sup> species. Use of metal L-edge XAS to assess a compound of uncertain formulation requires: 1) Placement of that data within the context of spectra offered by unambiguous calibrant compounds, preferably with the same coordination number and similar metal ligand distances. Such spectra assist in defining upper and/or lower limits for metal Z<sub>eff</sub> in the species of interest; 2) Evaluation of that data in conjunction with information from other physical methods, especially ligand K-edge XAS; 3) Increased care in interpretation if strong π-acceptor ligands, particularly CO, or π-donor ligands are present. The electron-withdrawing/donating nature of these ligand types, combined with relatively short metal-ligand distances, exaggerate the difference between formal oxidation state and metal Z<sub>eff</sub> or, as in the case of [W<sup>IV</sup>(mdt)<sub>2</sub>(CO)<sub>2</sub>], add other subtlety by modulating the redox level of other ligands in the coordination sphere
Vibrational and Electronic Structure of the α- and β-Naphthyl Radicals via Slow Photoelectron Velocity-Map Imaging
Dependence of Catalytic Activity on Driving Force in Solution Assays and Protein Film Voltammetry: Insights from the Comparison of Nitrate Reductase Mutants
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A variable temperature solid-state nuclear magnetic resonance, electron paramagnetic resonance and Raman scattering study of molecular dynamics in ferroelectric fluorides
The local nuclear and electronic structures and molecular dynamics of the ferroelectric lattice in selected geometric fluorides (BaMgF(4), BaZnF(4), BaMg(1-x)Mn(x)F(4) and BaMg(1-x)Ni(x)F(4); x = 0.001 and 0.005) have been investigated. The (19)F and (25)Mg isotropic chemical shift delta(iso), (25)Mg quadrupolar coupling constants (C(q)) and asymmetry parameters (eta) reflect the geometry of the coordination spheres. The zero-field splitting parameters vertical bar D vertical bar and vertical bar E vertical bar are consistent with distorted axial symmetry (low temperatures) and nearly rhombic symmetry (high temperatures) of octahedral Mn(2+) coordination. The high resolution of the nuclear magnetic resonance, electron paramagnetic resonance and phonon spectra are consistent with the highly ordered crystallographic structure. Combined multi-technique data evidence the subtle discontinuous changes in the temperature dependences of vertical bar D vertical bar and vertical bar E vertical bar, isotropic chemical shifts delta(iso) and signature parameters of Raman bands and suggest a discontinuous structural distortion of the fluoride octahedra. The temperature at which this change occurs depends on the ionic radius of the central ion of the octahedral site and is estimated to be similar to 300 K for Zn(2+) fluorides and similar to 240 K for Mg(2+) fluorides. This geometrical distortion modifies the lattice dynamics and originates from the rotation of the fluoride octahedra around a new direction approximately perpendicular to that related to the paraelectric-ferroelectric phase transition
Preparation and structural studies of [Mo(N3S2){R-2(O)PNP(S)R-2}(2)]
The compounds [Mo(N3S2){Ph-2(O)PNP(S)Ph-2}(2)] 1 [Mo(N3S2){Pr-1(2)(O)PNP(S)Pr-1(2)}(2)] 2 have been synthesised by treating [MoCl3(N3S2)] With KN(PPh2S)(2) or KN((PPh2S)-Ph-1)(2). X-Ray structures of 1 and 2 have been solved. On complexation, the MoN3S2 ring remained planar, but the Mo(OPNPS)(2) rings are puckered.</p
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