New V2O5 polymorphs have risen to prominence as a result of their open framework structures, cation intercalation properties, tunable electronic structures, and wide range of applications. The application of these materials and the design of new, useful polymorphs requires understanding their defining structure-property relationships. We present a characterization of the band gap and electronic structure of nanowires of the novel ζ-phase and the orthorhombic α-phase of V2O5 using X-ray spectroscopy and density functional theory calculations. The band gap is found to decrease from 1.90 ± 0.20 eV in the α-phase to 1.50 ± 0.20 eV in the ζ-phase, accompanied by the loss of the α-phase's characteristic split-off dxy band in the ζ-phase. States of dxy origin continue to dominate the conduction band edge in the new polymorph but the inequivalence of the vanadium atoms and the increased local symmetry of [VO6] octahedra results in these states overlapping with the rest of the V 3d conduction band. ζ-V2O5 exhibits anisotropic conductivity along the b direction, defining a 1D tunnel, in contrast to α-V2O5 where the anisotropic conductivity is along the ab layers. We explain the structural origins of the differences in electronic properties that exist between the α- and ζ-phase.
By studying Fe-doped ZnO pellets and thin films with various x-ray spectroscopic techniques, and complementing this with density functional theory calculations, we find that Fedoping in bulk ZnO induces isovalent (and isostructural) cation substitution (Fe 2+ → Zn 2+ ).In contrast to this, Fe-doping near the surface produces both isovalent and heterovalent substitution (Fe 3+ → Zn 2+ ). The calculations performed herein suggest that the most likely defect structure is the single or double substitution of Zn with Fe, although, if additional oxygen is available, then Fe substitution with interstitial oxygen is even more energetically favourable.Furthermore, it is found that ferromagnetic states are energetically unfavourable, and ferromagnetic ordering is likely to be realized only through the formation of a secondary phase (i.e.ZnFe 2 O 4 ), or codoping with Cu.
Herein
we systematically study a range of dopants (Cr, Fe, Ni,
Cu, and an MnCo alloy) in ZnO and TiO2 using several X-ray
spectroscopic techniques. We identify the dopant’s local environment
and interaction with the host lattice by employing crystal field multiplet
calculations and hence clarify their potential applicability for spintronic
technologies. Our density functional theory (DFT) calculations predict
a decreasing probability of direct cation (Zn/Ti) substitution by
dopant atoms as atomic number increases, as well as a much greater
likelihood of metallic clustering in TiO2. Our spectroscopic
measurements confirm that in all cases, except Mn, metallic clusters
of dopant atoms form in the TiO2 crystal lattice, thus
making it unfit for spintronic capabilities. On the other hand, in
ZnO, the dopants substitute directly into zinc sites, which is promising
for spintronic technologies.
Using inelastic X-ray scattering beyond the dipole limit and hard X-ray photoelectron spectroscopy we establish the dual nature of the U5felectrons in UM2Si2(M = Pd, Ni, Ru, Fe), regardless of their degree of delocalization. We have observed that the compounds have in common a local atomic-like state that is well described by the U5f2configuration with theΓ1(1)andΓ2quasi-doublet symmetry. The amount of the U 5f3configuration, however, varies considerably across the UM2Si2series, indicating an increase of U 5f itineracy in going from M = Pd to Ni to Ru and to the Fe compound. The identified electronic states explain the formation of the very large ordered magnetic moments inUPd2Si2andUNi2Si2, the availability of orbital degrees of freedom needed for the hidden order inURu2Si2to occur, as well as the appearance of Pauli paramagnetism inUFe2Si2. A unified and systematic picture of the UM2Si2compounds may now be drawn, thereby providing suggestions for additional experiments to induce hidden order and/or superconductivity in U compounds with the tetragonal body-centeredThCr2Si2structure.
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