Subtleties of structure and bonding in TaSSe and TaNbS solid solutions
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Substitution of selenium by sulfur and tantalum by niobium into the 1x[Ta5Ta] chains characteristic of Ta3S2 and Ta2S is attempted in an effort to understand the structural diversity of a metal-rich chalcogenides. Neither Ta2S nor Ta3S2 incorporates a significant amount of selenium, while Ta2Se-like structures are found to persist in Ta2S1-xSex for 0.2x1.0, a and c progressively increasing with x. The Ta2Se-like structure is stable to annealing at temperatures 1000C for 0.5x1.0, and compositions with x0.5 disproportionate to Ta3S2, Ta6S, and Ta1-xSe2 on annealing. Both our work and research completed in Franzen's laboratories show that niobium substitution into the 1x[Ta5Ta] chains does not occur to any large extent, instead layered Ta2Se-like structures (M4S2 and M5S2) are stabilized. At the composition Ta2-xNbxS (x=0.6) as-cast samples are virtually single phase, adopting a Ta2Se-like structure. A single crystal structure determination for a crystal with composition Ta1.4Nb0.6S was carried out: space group P4/nmm (No. 129), a=3.339(1), c=9.089(7) , V=101.33(9) 3, Z=2. While electronic structure calculations nicely rationalize the metal-metal bonding in any of these structures, the ability to predict which structures will be stabilized for which systems remains out of reach. 1992 Academic Press, Inc.
