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- Nb1.09S2 and 2H-NbSe2) are reported: the sulfide compound that crystallizes in the R3m space group (a = 3.3297(6) Å, c = 17.869(5) Å) has been refined to a RF value of 2.63%, while the selenide compound crystallizing in the P63/mmc space group (a = 3.4425 (5) Å, c = 12.547 (3) Å) has been refined to a RF value of 1.33%. The sulfide derivative is nonstoichiometric with extra Nb atoms statistically distributed over the vacant octahedral sites [site occupancy factor (s.o.f.) ≈ 9%] within the van der Waals gap separating two adjacent NbS2 sandwiches (where Nb is in a trigonal prismatic coordination). On the contrary, the selenide derivative appears perfectly stoichiometric.