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

Abstract

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.