SYNTHESIS AND CRYSTAL-STRUCTURE OF A NOVEL HEXAGONAL MODIFICATION OF AL2S3 WITH 5-COORDINATED ALUMINUM

A new hexagonal high temperature modification of Al2S3 could be prepar ed by chemical vapour transport with iodine (860 --> 750-degrees-C) or by annealing of alpha-Al2S3 at 550-degrees-C. According to the single crystal X-ray structure determination the novel form of Al2S3 crystal lizes in space group P6(1) (No. 169) with a = 6.491(1), c = 17.169(4) angstrom, V = 626.5 angstrom3, Z = 6; R = 0.0253. In this modification one half of the aluminum atoms are tetrahedrally coordinated [d(Al-S) : 2.226-2.267 angstrom], whereas the other half are in trigonal bipyra midal coordination of five S atoms with bond lengths of 2.272-2.315 an gstrom (equatorial) and 2.495-2.521 angstrom (axial). Aluminum in AlS5 coordination is observed for the first time in this compound. The cry stal structure is isotypic to In2Se3 and AlInS3. In addition, results of a refinement of the alpha-Al2S3 crystal structure are reported whic h were obtained on crystals prepared also by chemical vapour transport with iodine.