Sr₁₃□NbAs₁₁ and Eu₁₃□NbAs₁₁ – Defect Variants of the Ca₁₄AlSb₁₁ Structure with asymmetric [As₃]⁷⁻ anions

The novel compounds Sr 13 NbAs 11 and Eu 13 NbAs 11 have been synthesized from SrAs, Eu 5 As 4 , Sr, Nb and As in niobium ampoules at 1173–1273 K. The tetragonal tI 200 phases are defect variants of the Ca 14 AlSb 11 structure (space group I 4 1 / acd (no. 142); Sr 13 □NbAs 11 : a = 1649.8(2) and c = 2214.1(3); Eu 13 □NbAs 11 : a = 1632.9(8) and c = 2197.3(8) pm; Z = 8). The structures are built from the cations Sr 2+ , and Eu 2+ , respectively, and from the anions [NbAs 4 ] 7− , As 3− , and the linear polyanion [As 3 ] 7− . This polyanion (isosteric to I 3 − ) is asymmetric with d(AsAs) = 273.0 and 346.0 pm (Sr) and 274.7 and 335.6 pm (Eu), respectively. The bond lengths in the tetrahedral anions are d(NbAs) = 250.8 and 251.1 pm. The complete structural arrangement is related to that of Cu 2 O by forming two interpenetrating networks. The oxygen atoms are substituted by niobium centered As 4 tetrahedra, and the Cu atoms are substituted by As 6 octahedra (centered by Sr, Eu). The central As atoms of the polyanions connect the nets. Both As networks are enveloped by the remaining cations forming cubes, tetragonal antiprisms and capped trigonal prisms.