Volume 215, Issue 11 - 11

Directional Compton profiles of Li metal measured with ultra-high momentum space resolution (0.02 a.u.) were analysed in terms of their reciprocal form factors B ( R ) at lattice translation vectors R , in order to investigate the influence of electron correlation on the occupation number density. Differences between the B ( R ) calculated from measured Compton profiles and non-correlation-corrected calculated ones are confronted with corresponding quantities deduced from four different models of a correlated free electron momentum density. This comparison clearly indicates that the origin of the discrepancy between Compton profiles calculated on the basis of the local density approximation and the experiment must be traced back to correlation induced deviations of the occupation number density from its simple uncorrelated form and stresses the great sensitivity of the correlation correction (Lam–Platzman correction) to models of the correlated free electron momentum density. The pressing need of a critical evaluation of the correlation induced changes of the occupation number density of simple metals is emphasized.

New cobalt disilicide high resolution Compton scattering measurements have been performed at ESRF (European Synchrotron radiation facility) in France. In a first step, experimental results are compared with calculated results, obtained from a self consistent linear muffin tin orbitals band structure calculations. The second step consists in studying three dimensional electron momentum density (EMD) distribution – 3D map – in CoSi 2 (experimental data from ESRF) mainly by the Maximum Entropy Method (MEM). Detailed comparison of the results obtained for a 3-dimensional electron momentum density reconstruction by means of MEM and the Fourier Transformations (FT) is carried out for CoSi 2 , and compared with the theoretical 3D-map. It is shown that both methods of analysis deliver essentially similar results. For the first time, the MEM was applied with theoretical distribution taken as a prior, and with the experimental momentum resolution built into the reconstruction procedure. Therefore the experimental Compton profiles needed no deconvolution prior to the reconstruction. Also the error maps of the reconstructed EMD are estimated for the first time and it is shown that the statistical errors are smaller than systematical uncertainties. In order to explain observed differences between experimental data and theory, more theoretical work has to be done.

Compton profiles of polycrystalline Cr-Fe-Mn compounds were measured at room temperature using 60-keV gamma radiation from a  241 Am isotope source. Four difference profiles for the samples of varying composition were formed from the experimental data. The difference profiles suggest the presence of charge transfer from delocalised 4 s -like to more localised 3 d -type orbitals during alloying. The value of the transfer is well modelled, using the APW-type approach, when an antiferromagnetic spin arrangement is considered. However, this effect is different for the ferromagnetic case.

Local environments of divalent europium ions in β-lattice of barium magnesium aluminate, BaMgAl 10 O 17 (BAM) have been investigated using a recently developed ab initio band structure method. This method is a variant of the full potential LMTO method. The reliability of this method for calculating electric field gradient (EFG) was tested in the case of aluminum oxide in corundum structure. The calculated EFGs at both cation and anion sites compare satisfactorily with those from other ab initio methods and from recent measurements. The nuclear quadrupole coupling constants for Eu 2+ calculated by this method at various possible sites in BAM, when compared to recently measured 151 Eu Mössbauer spectroscopy results, suggest three locations for Eu 2+ : a Beevers–Ross site, a mid-oxygen site and an anti-Beevers–Ross site. Energetically, the anti-Beevers–Ross site appears to be more stable than the other two sites.

The non-isothermal decomposition of NiC 2 O 4 ·2H 2 O was investigated under a dynamic inert atmosphere using a temperature-programmed X-ray diffractometer (TP-XRD) and a thermogravimetric analyzer coupled to a Fourier-transform infrared spectrometer (TGA-FTIR). Both solid and gaseous products were analyzed in situ. The XRD pattern of the dehydration product did not match data for NiC 2 O 4 reported in the JCPDS file. The thermal decomposition of the dehydration product (NiC 2 O 4 ) took place in two sub-steps, both of which involved CO 2 evolution. Metallic nickel was the final product.

Several monochromators, which are based on the use of cylindrically bent perfect Si crystals, have been constructed at the High Energy X-ray Scattering beamlines of the ESRF. The monochromators provide different focusing conditions, and the energy band-passes are optimized for the needs of different experiments. Formulas are given for calculation of the focal distances, reflectivity curves, and energy distributions. The lay-out of the beamlines follows the Troika concept, where the radiation fan is either split in 3 beams, or the central beam is utilized successively by semi-transparent monochromators to serve three experimental stations simultaneously. The radiation sources are a 7-period permanent magnet asymmetric wiggler and a superconducting wavelength shifter. The critical energies are 45 keV and 96 keV, respectively. The lowest operation energy is 30 keV, and transmission type monochromators have been used up to 1 MeV photon energies. Typical X-ray flux at the sample is 10 12  ph/s at energies below 150 keV. The monochromator crystals are partly immersed in In-Ga eutectic, which provides thermal contact between the crystal and the water-cooled support structure. Bending mechanisms include flexible springs, so that the actuator motions are several orders of magnitude larger than the crystal motions. This eliminates the effects of the thermal drifts of the support structures.

An approximate theory for the scattering of X-rays by free atoms in the resonance region for both elastic and inelastic scattering is developed based on the exact results through first order in perturbation theory of non relativistic X-ray scattering obtained by Gavrila and Costescu [Phys. Rev. A 2 , 1752 (1970)]. The A · p corrections for the elastic scattering of X-rays from the ground state of atomic hydrogen are evaluated numerically in the resonance region and the results are compared with the predictions of the optical theorem and a dispersion relation using the exact results for the photo absorption spectrum. The possibility of carrying out a polarization dependent experiment to observe the contribution to the A · p correction from non dipole excitations is analyzed and predicted to be feasible with currently available X-ray sources.