Fig. 2 Total density of states (left) and Nb-Nb owxlap population (right) for NbiS21L4. ( + ) : bonding Nb-Nb interactions; ( -): antibonding Nb-Nb interactions. Dashed line indicates [he Fermi level (hlghest occupied orbital), which occurs at ;I small energy gap (0.2 eV) in the DOS that is not completely visible on this scale.
cule, which indicates a do configuration for these Nb atoms. If we compare these energies with those for a Irons-vertex-sharing chain of NbI, octahedra,[*] which give rise to a broad N b 4d band by significant through-bond coupling with the bridging I atoms, this band would overlap with the Nb-Nb bonding band in the Nb,SI, layer. Our calculations strongly suggest, therefore, that the monomers form so as to optimize Nb-Nb interactions within the two-dimensional framework, but with little charge transfer between the two components. Thus, van der Waals and other London dispersion forces are the dominant interactions holding these two structural components together.
Similar calculations on the alternative structure of Nb,SI, expected from earlier work on ternary Nb,YX, compounds in this family['91 reveal that the bicapped Nb, cluster is energetically less favorable than the monocapped alternative.[2o1 All of our attempts to remove the NbI, molecules from Nb,S,I,, have resulted in the formation of NbI,, I,, and the Nb,SI, trigonal variant. Thus, a significant structural synergism exists between these two components, each of which is unstable with respect to either another structure (Nb,SI,) or towards decomposition (NbI,), but are stabilized when they "cocondense."
Experinwntal Procedure
Elemental N b (foil. A h : washed HF:HNO, and dried under vacuum). S. and I (sublimed; Alfa) in the molar ratio 7.2: 19 were sealed under Ar in a quart/ ampule (@ = 10 mm: I = 60 inm) and heated at 1100 K for 2 d. The product forms black. needle-shaped crystals. Chemical analysis with the Volhard method [21] gave I : 75.0% and S : 2.0%. N b analysir (a, Nb,O,) gave N b : 20.2%. X-ray powder diffraction using an Enraf-Nonius Guinier camera indicates the phase NbiS,I,, is accompanied b) a small amount of a hexagonal phase we identify as Nb,Sl, [19]. Magnetic susceptibility measurements show a small Curie-Weiss contribution (0.018 cm' K m o l ~ ' ) from a paramagnetic impurity. Two-probe electrical resistivity measurements indicated p(300 K ) > lo4 Cl cm.