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NE Ghermani

Publications and source records attributed to NE Ghermani.

4 recordsLinked to original sources
PubMed1999

The use of CCD area detectors in charge-density research. Application to a mineral compound: the alpha-spodumene LiAl(SiO3)2.

X-ray diffraction data sets collected on both Nonius and Siemens (Bruker) goniometers equipped with charge-coupled device (CCD) area detectors have been tested for the electron-density determination of the aluminosilicate mineral compound alpha-spodumene LiAl(SiO(3))(2), aluminium lithium silicon oxide. Data collection strategies, reflection intensity peak integration methods and experimental error estimates are different for the two instruments. Therefore, the consistency and quality of the two types of CCD measurements have been carefully compared to each other and to high-resolution data collected on a conventional CAD-4 point-detector diffractometer. Multipole density model refinements were carried out against the CCD data and the statistical factors analysed in terms of experimental weighting schemes based on the standard uncertainties of the diffraction intensities derived by the Nonius and Siemens software programs. Consistent experimental electron-density features in the Si-O-Si and Si-O-Al bridges were found from both CCD data sets. The net atomic charges obtained from the kappa refinements against each CCD data set are also in good agreement and quite comparable with the results of the conventional CAD-4 experiment.

Journal Article↗
PubMed1999

Molecular fragment electric moments derived from the fit of the experimental electrostatic potential. Application to the water molecule.

The electrostatic potential is a multicenter property that can be expressed as a sum of the contributions of electric moments located at each atomic site of a molecule. Independently of the model used to generate the electrostatic potential around the system, these atomic moments can be accurately obtained by the fit of this physical property outside the van der Waals envelop. However, the larger the system, the greater the number of parameters. In this study a way is proposed to reduce the number of centers in the representation of the electrostatic potential which becomes a sum of fragment contributions rather than atomic ones. A sample of six water molecules in different crystal environments was chosen to discuss the derived values of the electric moments referred to the molecular center of mass.

Journal Article↗
PubMed1999

Electron density distribution and Madelung potential in alpha-spodumene, LiAl(SiO3)2, from two-wavelength high-resolution X-ray diffraction data.

The electron density distribution in alpha-spodumene, LiAl(SiO(3))(2), was derived from high-resolution X-ray diffraction experiments. The results obtained from both Mo Kalpha- and Ag Kalpha-wavelength data sets are reported. The features of the Si-O and Al-O bonds are related to the geometrical parameters of the Si-O-Al and Si-O-Si bridges on the one hand and to the O.Li(+) interaction on the other. Kappa refinements against the two data sets yielded almost the same net charges for the Si (+1.8 e) and O (-1.0 e) atoms in spodumene. However, the Al net charge obtained from the Ag Kalpha data (+1.9 e) is larger than the net charge derived from the Mo Kalpha data (+1.5 e). This difference correlates with a more contracted Al valence shell revealed by the shorter X-ray wavelength (kappa = 1.4 for the Ag Kalpha data set). The derived net charges were used to calculate the Madelung potential at the spodumene atomic sites. The electrostatic energy for the chemical formula LiAl(SiO(3))(2) was -8.60 e(2) Å(-1) (-123.84 eV) from the net charges derived from the Ag Kalpha data and -6.97 e(2) Å(-1) (-100.37 eV) from the net charges derived from the Mo Kalpha data.

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