CHALCOGEN SUBSTITUTION IN THE TA4P4S29 TUNNEL STRUCTURE - SYNTHESIS AND STRUCTURE OF TAPS6SE

A compound with a composition close to TaPS//6Se was prepared either from the elements or from TaPS//6 and Se heated respectively at 630 and 460 degree C in evacuated silica tubes for 20 days. The crystal symmetry is monoclinic, space group P2, with the following cell parameters: A equals 15. 657(2) angstrom, b equals 6. 8714(8) angstrom, c equals 22. 273(3) angstrom, beta equals 135. 09(1) degree , V equals 1691. 7(5) angstrom**3, and Z equals 8. The structure determination made from 3238 reflections and 244 parameters yielded a reliability factor of R equals 0. 050. TaPS//6Se atomic arrangement is very similar to that of Ta//4P//4S//2//9, with the same basic tunnel framework in which, instead of ordered sulfur helices, selenium chains, partially substituted by sulfur, are inserted. The chains of composition (Se//8S//2) present a periodicity of 2b, but owing to the existence of four different statistically distributed chains, no superstructure is observed.