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(SiO3)(2), was de rived from high-resolution X-ray diffraction experiments. The results obtai ned 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 par ameters 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 (+1.8e) and O (-1.0e) 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 r evealed by the shorter X-ray wavelength (kappa = 1.4 for the Ag K alpha dat a set). The derived net charges were used to calculate the Madelung potenti al at the spodumene atomic sites. The electrostatic energy for the chemical formula LiAl(SiO3)(2) was -8.60 e(2) Angstrom(-1) (-123.84 eV) from the ne t charges derived from the Ag K alpha data and -6.97 e(2) A(-1) (-100.37 eV ) from the net charges derived from the Mo K alpha data.