RESUMO
The binary niobium sulfide NbS4 was synthesized as a crystalline phase. We showed that NbS4 can be formed from Nb metal, from defect niobium sulfide Nb1.14S2, or from some other niobium dichalcogenides in reactions with excess sulfur in an evacuated ampule at 440 °C. The crystal structure of NbS4 (monoclinic space group C2/c, a = 13.126(2) Å, b = 10.454(1) Å, c = 6.951(1) Å, ß = 111.939(5)°) is a packing of infinite chains {NbS4}1∞, analogous to VS4. In the chains, Nb atoms are in a tetragonal-antiprismatic coordination of sulfur atoms of disulfide groups (S2)2-; short Nb···Nb contacts (2.896 Å) alternating with longer ones (3.278 Å) appear within the chains at 150 K. According to density functional theory calculations, NbS4 is a thermodynamically stable compound, a nonmagnetic semiconductor. NbS4 is a new member of the family of quazi-one-dimensional compounds, group 5 metal polychalcogenides, well-known for their interesting electrophysical properties. The synthesis and crystal structure as well as the thermal stability and lattice dynamics of NbS4 are discussed here.
RESUMO
The first vanadium selenoiodide V4O(Se2)4I6·I2 was synthesized at a moderate temperature of 220 °C from V, Se, I2, and water. Its crystal structure (tetragonal space group P42/nbc, a = 11.838(1) Å, c = 18.689(1) Å) contains O-centered vanadium(IV) tetranuclear fragment [V4(µ4-O)(µ2-Se2)4(µ2-I)2I4], where the edges of the distorted tetrahedron V4 are bridged by four diselenide (Se2)2- and two iodide ligands; four terminal iodides coordinate V atoms additionally. This type of complex is known for Ti, Nb, and Ta but is new for vanadium. Magnetic susceptibility measurements of V4O(Se2)4I6·I2 showed four unpaired electrons on vanadium atoms at room temperature and drop of the effective magnetic moment at cool down, presumably due to partial electron pairing. Probability of this transition to the diamagnetic state is in accord with the calculated electronic structure.