Synthesis and structure of Na<inf>5</inf>Sr[NbAs<inf>4</inf>] and K<inf>5</inf>Ba[NbAs<inf>4</inf>]

The two novel tetraarsaniobates(V) Na5Sr[NbAs4] and K5Ba[NbAs4] have been synthesized from the powders of As, Nb and binary arsenides Na3As, K3As, SrAs or BaAs in sealed niobium ampoules and characterized by single crystal X-ray crystallography. The compounds crystallize in the hP22 type of Na6[ZnO4] (space group P63mc (No. 186); Na5Sr[NbAs4], a = 955.7(1) pm, c = 751.7(2) pm; K5Ba[NbAs4], a = 1039.6(2) pm, c = 809.7(3) pm; Z = 2). The bond lengths d(Nb-As) in the [NbAs4]7- anions range from 248.0(4) to 248.8(1) pm and from 249.2(3) to 250.3(1) pm in Na5Sr[NbAs4] and K5Ba[NbAs4] respectively.