First discovery and structural characterization of a new compound in Al-Si-O-C system

A quaternary oxycarbide, [Al16.77(5)Si1.23(5)]Sigma(18)[O3.04(9)C10.96(9)]Sigma(14), has been for the first time discovered in the Al-Si-O-C system. The crystal structure was characterized by X-ray powder diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy (EDX). The atom ratios [Al:Si] were determined by EDX, and the initial structural model was derived by the direct methods. The structural parameters as well as the atom ratios [O:C] were determined by the Rietveld method. The crystal is monoclinic (space group C2/m, Z = 1) with lattice dimensions a = 0.57404(1) nm, b = 0.331435(5) nm, c = 1.92410(2) nm, beta = 90.036(1)degrees and V = 0.366076(9) nm(3). The final structural model showed the positional disordering of Al/Si sites. The validity of the split-atom model was verified by the three-dimensional electron density distribution, the structural bias of which was reduced as much as possible using the maximum-entropy methods-based pattern fitting (MPF). The reliability indices calculated from the MPF were R-wp = 4.20% (S = 1.14), R-p = 3.09%, R-B = 0.92% and R-F = 1.05%. The crystal was an inversion twin with nearly the same twin fraction. (C) 2009 Elsevier Inc. All rights reserved.