SYNTHESIS, STRUCTURE, AND PROPERTIES OF T HE TETRAARSENIDOMETALLATES(V) M(7)[TAS4] (M=K, RB T=NB, TA)

The tetraarsenidometallates(V) M(7)[TAs4] (M = K, Rb; T = Nb, Ta) have been prepared from RbAs, KAs, Rb3As, K3As, and Nb or Ta in sealed Nb( Ta) ampoules at T = 1100 K. They crystallize in a new structure type o P24 (Pmn2(1), no. 31); K-7[NbAs4]: a = 1019.2(2)pm, b = 916;2(2)pm, c = 830.6(1)pm; K-tau[TaAs4]: a = 1017.3(2)pm, b = 915.5(2)pm, c = 830.5 (2)pm; Rb-7[NbAs4]: a = 1059.2(4)pm, b = 952.8(4)pm, c = 860.4(4)pm; Z = 2 formula units per unit cell). The compounds form dark red crystal s and they are sensitive against air and moisture. They are semiconduc tors with E(g) = 1.80 eV. The thermal decomposition in dynamical vacuu m gives evidence for the existance of K4TAs3 and K2TAs2 (T = Nb, Ta). Main structural units are polar oriented tetrahedra [TAs4] with (d) ov er bar(T-As) = 252.2(1)pm; 251.3(1)pm; 253.0(4)pm, respectively. The A s atoms are trigonal prismatically coordinated by M and T atoms. These trigonal prisms form anionic and cationic layers (2)(infinity)[M(4)As (2)](2-) and (2)(infinity)[M(3)TAs(2)](2+) alternating along the b axi s. The structure is comparable with that of Co2P and can be described as a stuffed shear variant of the Na-6 square ZnO4 type of structure.