Abstract
The two title compounds were prepared by direct reactions of the corresponding elements at high temperature, and their structures were determined from single-crystal X-ray diffraction data. The structure of K(38)Nb(7)As(24) (orthorhombic; Cmcm; Z = 4; a = 10.4974(6), b = 23.915(2), c = 36.046(2) A) comprises isolated tetrahedra of NbAs(4) and two types of dimers of edge-sharing tetrahedra: dimers containing only Nb(V), [Nb(V)2As(6)](8-), and mixed-valence dimers with both Nb(IV) and Nb(V), [Nb(IV)Nb(V)As(6)](9-). The structure of Cs(9)Nb(2)As(6) (orthorhombic; Pbca; Z = 8; a = 17.5848(7), b = 16.940(2), c = 18.183(4) A) contains only the latter dimers. Magnetic measurements showed Curie-Weiss paramagnetic behavior for both compounds consistent with one unpaired electron/mixed-valence dimer. Cs(9)Nb(2)As(6) exhibits also an antiferromagnetic transition at about 36 K. The two compounds are the first mixed-valence (of class III) transition-metal Zintl phases.
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