Acta Crystallographica Section B

Structural Science

Volume 55, Part 3 (June 1999)

research papers

Acta Cryst. (1999). B55, 273-284 [ doi:10.1107/S0108768198013536 ]

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

S. Kuntzinger and N. E. Ghermani

Abstract: The electron density distribution in [alpha] -spodumene, LiAl(SiO₃)₂, was derived from high-resolution X-ray diffraction experiments. The results obtained from both Mo K [alpha] - and Ag K [alpha] -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 OLi⁺ 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 K [alpha] data (+1.9 e) is larger than the net charge derived from the Mo K [alpha] 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 K [alpha] 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(SiO₃)₂ was -8.60 e² Å^-1 (-123.84 eV) from the net charges derived from the Ag K [alpha] data and -6.97 e² Å^-1 (-100.37 eV) from the net charges derived from the Mo K [alpha] data.