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2003
Conference Paper
Titel
Determination of the morphology of HNIW-phases by the attachment energy method
Abstract
Crystallographic computing techniques using a SGI Octane station and program Cerius 4.2 from Accelrys allow us to predict shape and habit of organic crystals. Force field methods using the QEq charge distribution and the Morph_Momany 1.1 force field were applied to investigate the morphology of the high explosive Hexanitrohexaazaisowurtzitan (HNIW). HNIW (C6H6N12O12) is a nitramine with a cage structure, and it crystallizes in 4 polymorphs at ambient temperature and pressure. First the morphologies of the HNIW crystals were approximated by a geometric approach according to Bravais-Friedel-Donnay-Harker (BFDH). The BFDH method uses the crystal lattice and the symmetry to calculate a list of possible growth faces and their relative growth rates. Generally, observed crystal morphology is found to be dominated by lower order faces. In a second step attachment energies and growth rates were calculated for selected crystal faces, and refined growth rates and center to face distances were assigned to each face. The attachment energy Eatt is defined as the energy release on the attachment of a growth slice to a growing crystal surface. Comparison of the calculated habits with SEM pictures of HNIW phases crystallized from different solutions shows a more accurate morphology simulation with the attachment energy method than with the BFDH method.