A new mixed group 5 metal selenide, Nb1.41V0.59Se9

The new mixed-metallic phase, niobium vanadium nonaselenide, (Nb2-xVx)Se9 (0.18≤ x ≤ 0.59) is isostructural with monoclinic V2Se9. The structure is composed of chains of bicapped trigonal–prismatic [MSe8] units. The metal (M) site is occupied by statistically disordered Nb [0.706 (5)] and V [0.294 (5)] atoms. Two trigonal prisms are linked by sharing a rectangular face composed of two Se2 2− pairs. Through three edging and capping Se atoms, the chains are extended along [101]. The chain shows alternating short [2.8847 (7) Å] and long [3.7159 (7) Å] M—M distances. The structure shows a wide range of Se—Se interactions. In addition to the Se2 2− pairs of the rectangular face, an intermediate Se⋯Se separation [2.6584 (5) Å] is found. The amount of each metal can vary, [(Nb2-xVx)Se9, 0.18 ≤ x ≤m 0.59] and they seem to form a random substitutional solid solution. The M—M distances increase gradually by increasing the amount of Nb atoms. The classical charge-balance of the compound can be described as [M 4+]2[Se2 2−]2[Se5 4−].


Related literature
For related group 5 metal chalcogenide triclinic Nb 2 Se 9 structures, see: Meerschaut et al. (1979);Sunshine & Ibers (1987). For the synthesis and structures of related group 5 metal monoclinic V 2 Se 9 chalcogenides, see: Furuseth & Klewe (1984 (Meerschaut et al., 1979;Sunshine & Ibers, 1987) and monoclinic V 2 Se 9 (Furuseth & Klewe, 1984) have been reported to have one-dimensional chain structures. These compounds share the same one-dimensional chain structure. However they show different packing of the chains and the twofold rotational symmetry is not observed in Nb 2 Se 9 . During our search for new group 5 metal chalcogenides, we have found a new mixed-metallic phase, (Nb 2 -x V x )Se 9 (0.18≤x≤0.59) and here we report the synthesis and crystal structure of (Nb 1.41 V 0.59 )Se 9 .
The title compound is isostructural with monoclinic V 2 Se 9 (Furuseth & Klewe, 1984). The structure is composed of The shortest interchain Se-Se distance is 3.5479 (8) Å and thus there is no strong bonding interaction among the chains.
The chain shows alternating short (2.8847 (7) Å) and long (3.7159 (7)  increasing the amount of Nb atoms (Fig.3) of Nb, V, and Se. Both X-ray diffraction studies and quantitative XRF analysis indicated that stoichiometries of each metal vary considerably for crystals even from the same reaction tube. The average Nb: V ratio for many crystals is 78: 22.

Refinement
The disordered nature of the metals in the title compound was checked by refining the anisotropic displacement parameters (ADPs). When the structure was refined assuming Nb 2 Se 9 and V 2 Se 9 , the displacement parameters of the metal sites were very large and small, respectively. In both cases the reliability indices were high (wR2 > 0.098). With the mixed-metal model, the ADPs of the metal atoms are comparable with those of the other atoms and the residuals were reduced significantly (wR2 = 0.051). The Se atoms were refined anisotropically. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > 2σ(F 2 ) is used only for calculating Rfactors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq Occ.