9.
Synthesis and Crystal Structure of a Novel Hexagonal Modification of Al
2S
3 with Five-coordinated Aluminum A new hexagonal high temperature modification of Al
2S
3 could be prepared by chemical vapour transport with iodine (860 → 750°C) or by annealing of α -Al
2S
3 at 550°C. According to the single crystal X-ray structure determination the novel form of Al
2S
3 crystallizes in space group P 6
1 (No. 169) with a = 6.491(1), c = 17.169(4) Å, V = 626.5 Å
3, Z = 6; R = 0.0253. In this modification one half of the aluminum atoms are tetrahedrally coordinated [d(Al? S): 2.226–2.267 Å], whereas the other half are in trigonal bipyramidal coordination of five S atoms with bond lengths of 2.272–2.315 Å (equatorial) and 2.495–2.521 Å (axial). Aluminum in AlS
5 coordination is observed for the first time in this compound. The crystal structure is isotypic to In
2Se
3 and AlInS
3. In addition, results of a refinement of the α -Al
2S
3 crystal structure are reported which were obtained on crystals prepared also by chemical vapour transport with iodine.
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