Crystal structure studies of the 3R-Nb1.09S2 and the 2H-NbSe2 compounds: correlation between nonstoichiometry and stacking type (= polytypism)

Crystal structure determinations of two niobium dichalcogenides (3R- Nb1.09 S2 and 2H-NbSe2) are reported: the sulfide compound that crystallizes in th e R3m space group (a = 3.3297(6) Angstrom, c = 17.869(5) Angstrom) has been refined to a R-F value of 2.63%, while the selenide compound crystallizing in the P6(3)/mmc space group (a = 3.4425 (5) Angstrom, c = 12.547 (3) Angs trom) has been refined to a R-F value of 1.33%. The sulfide derivative is n onstoichiometric with extra Nb atoms statistically distributed over the vac ant octahedral sites [site occupancy factor (s.o.f.) approximate to 9%] wit hin the van der Waals gap separating two adjacent NbS2 sandwiches (where Nb is in a trigonal prismatic coordination). On the contrary, the selenide de rivative appears perfectly stoichiometric. (C) 2001 Elsevier Science Ltd. A ll rights reserved.