R. Boada

The hydration structure of Cu2+: more tetrahedral than octahedral?

A comprehensive multi-technique approach has been used to address the controversial question of the preferred geometric form of the Cu2+ aqua-ion hydration shell. A combination of H/D isotopic substitution neutron scattering and X-ray scattering has been used to refine atomistic models of 0.5 m and 2.0 m solutions of Cu(ClO4)2, that have also been constrained to simultaneously reproduce detailed local structure information about the cation environment obtained by X-ray Absorption spectroscopy. The adoption of the Empirical Potential Structure Refinement (EPSR) technique as a single unified analytical framework minimises the chances for biasing the result in favour of a specific pre-conceived outcome. The results are consistent with an average coordination for each Cu2+ ion of 4.5 ± 0.6 water molecules that matches the more recent picture of five-fold coordination in a 2.0 m solution, but interestingly this combined study highlights that the preferred local geometry of the ion sites is found to have a mixed character of tetrahedral, trigonal bipyramidal and octahedral components. A further point to note is that this new model adds support to a largely ignored result in the literature relating to the linear electric field effect induced g-shifts observed in the electron paramagnetic resonance spectra of glassy Cu2+ complexes (Peisach and Mims, Chem. Phys. Lett., 1976, 37, 307–310) that first highlighted the importance of tetrahedral distortions in the cations hydration shell structure.

X-ray magnetic circular dichroism measurements using an X-ray phase retarder on the BM25 A-SpLine beamline at the ESRF

The experimental set-up at SpLine (BM25A, ESRF) to measure XMCD by using a diamond X-ray phase retarder to obtain circularly polarized X-rays is described.

Disentanglement of magnetic contributions in multi-component systems by using X-ray magnetic circular dichroism at a single absorption edge

X-ray magnetic circular dichroism (XMCD) has become in recent years an outstanding tool for studying magnetism. Its element specificity, inherent to core-level spectroscopy, combined with the application of magneto-optical sum rules allows quantitative magnetic measurements at the atomic level. These capabilities are now incorporated as a standard tool for studying the localized magnetism in many systems. However, the application of XMCD to the study of the conduction-band magnetism is not so straightforward. Here, it is shown that the atomic selectivity is not lost when XMCD probes the delocalized states. On the contrary, it provides a direct way of disentangling the magnetic contributions to the conduction band coming from the different elements in the material. This is demonstrated by monitoring the temperature dependence of the XMCD spectra recorded at the rare-earth L(2)-edge in the case of RT(2) (R = rare-earth, T = 3d transition metal) materials. These results open the possibility of performing element-specific magnetometry by using a single X-ray absorption edge.

Rotation of X-ray polarization in the glitches of a silicon crystal monochromator

Rotation of X-ray polarization at the glitches of a monochromator composed of single crystals of silicon is observed. This effect can be explained by a model taking full account of the X-ray source, the effects of multiple-beam dynamical diffraction, and the coherent and Compton scattering from the sample.

Mercury capture on a supported chlorocuprate(II) ionic liquid adsorbent studied using operando synchrotron X-ray absorption spectroscopy

Mercury scrubbing from gas streams using a supported 1-butyl-3-methylimidazolium chlorocuprate(ii) ionic liquid ([C4mim]2[Cu2Cl6]) has been studied using operando EXAFS. Initial oxidative capture as [HgCl3]- anions was confirmed, this was then followed by the unanticipated generation of mercury(i) chloride through comproportionation with additional mercury from the gas stream. Combining these two mechanisms leads to net one electron oxidative extraction of mercury from the gas with increased potential capacity and efficiency for supported ionic liquid mercury scrubbers.

Temperature dependence of the Ho L2-edge XMCD spectra of Ho6Fe23.

An X-ray magnetic circular dichroism (XMCD) study performed at the Ho L2,3-edges in Ho6Fe23 as a function of temperature is presented. It is demonstrated that the anomalous temperature dependence of the Ho L2-edge XMCD signal is due to the magnetic contribution of Fe atoms. By contrast, the Ho L3-edge XMCD directly reflects the temperature dependence of the Ho magnetic moment. By combining the XMCD at both Ho L2- and L3-edges, the possibility of determining the temperature dependence of the Fe magnetic moment is demonstrated. Then, both microHo(T) and microFe(T) have been determined by tuning only the absorption L-edges of Ho. This result opens new possibilities of applying XMCD at these absorption edges to obtain quantitative element-specific magnetic information that is not directly obtained by other experimental tools.

Pressure dependence of magnetic states in Laves Phase RCo2 (R = Dy, Ho, and Er) compounds probed by XMCD

In order to probe pressure-induced modification in Co magnetic state in Laves phase RCo2 (R = Dy, Ho, and Er) compounds, we have measured X-ray magnetic circular dichroism (XMCD) under high pressure. The R L2-edge XMCD indicates that Co magnetic moment MCo gradually decreases as the pressure increases. The Co K-edge XMCD shows that the effect of R molecular field is significantly suppressed under high pressure. As a result, it is revealed that the pressure variation of MCo is closely associated with R-Co interaction. The Co magnetic state is separately discussed from that of R partner.

A Magnetic and MÖssbauer Spectral Study of the

We present here a magnetic and ⁵⁷Fe Mossbauer spectral study of the Lu(Al_1-xFe_x)₂ compounds (x=0.50, 0.75, and 1). At relatively high temperatures (T_H ~ 300 K for x=0.75 , and T_H ~ 70 K for x=0.50) a peak is observed in both zero field cooled magnetization, M(T), and low field ac susceptibility versus temperature, chi_ac(T), curves. Irreversible phenomena occur at temperatures below the maxima in M(T), and a clear dependence with the exciting frequency is observed in the chi_ac>(T) curves. Similar behavior is observed at lower temperatures (T_L ~ 45 K for x=0.75, and T_L ~ 10 K for x=0.50); however, the low temperature peak in chi_ac(T) is almost frequency independent. The combined analysis of magnetization data and ⁵⁷Fe Mossbauer spectroscopy suggests that the high temperature process corresponds to the apparition of short-range correlations between the magnetic moments, which lead to the formation of magnetic clusters, whereas the low temperature process corresponds to a freezing of the iron magnetic moments.

{\rm Lu}{({\rm Al}_{1-{\rm x}}{\rm Fe}_{\rm x})}_{2}

The magnetic polarization of the Lu 5d states through the Ho(1-x)Lu(x)(Fe(1-y)Al(y))(2) series has been studied by means of x-ray magnetic circular dichroism. A combined study of the dichroic signals performed at the Fe K-, Ho L(2)- and Lu L(2,3)-edges gives a complete picture of the polarization scheme at the conduction band. The results show that in the presence of competing localized magnetic moments, μ(Fe)(3d) and μ(Ho)(4f), the dichroic signal at the Lu site is mainly due to the Fe atoms, the effect of the magnetic rare-earth being negligible. Estimation of the spin and orbital components of the Lu(5d) induced magnetic moment have been obtained by applying the magneto-optical sum rules derived for x-ray magnetic circular dichroism.

Compounds

This work reports the x-ray absorption spectroscopy study of the chemical ordering in UCu5−xMx (M = Ni, Ag) compounds. The comparison between the experimental Cu K-edge XANES spectra and theoretical computations based on multiple-scattering theory shows that standard single-channel calculations are capable of reproducing the experimental spectra. On this subject, an extensive discussion is presented concerning the role of both the cluster size and the final state potential in obtaining a good reproduction of the experimental XANES spectra of these UCu5-based alloys. The agreement between the theoretical and experimental spectra points to the existence of crystallographic disorder in both UCu4Ag and UCu4Ni systems. Possessing the distinct low temperature electrical, magnetic and thermal properties exhibited by UCu4Ni, our results suggest that Ni doping must induce dramatic changes in the electronic structure, as confirmed by the thermopower measurements. These results point to the imbalance between the RKKY and Kondo interactions as the source of the NFL behaviour observed in UCu4Ni, thus supporting an interpretation of the NFL behaviour in terms of the Griffiths phase model.