Kristallstrukturuntersuchungen an Verbindungen des Typs A3(M,Nb)8O21 (A  Tl,Ba; M  Fe,Ni)

Crystal Structure Investigations of Compounds with the A₃(M, Nb)₈O₂₁-Type (A ? Tl, Ba; M ? Fe, Ni) Tl₃Fe_0,5Nb_7,5O₂₁ (A), a hitherto unknown phase of the A₃(M, Nb)₈O₂₁-type, and Ba₃Fe₂Nb₆O₂₁ (B), Ba₃Ni_1.33Nb_6,66O₂₁ (C) were prepared and investigated by single crystal X-ray technique. ((A): a = 9.145(1), c = 11.942(1) Å; (B): a = 9.118(2), c = 11.870(1) Å; (C) a = 9.173(3), c = 11.923(1) Å, space group D? P6₃/mcm, Z = 2). There is a statistic occupation of the M-positions by Nb⁵⁺ and Fe³⁺ or Nb⁵⁺ and Ni²⁺, respectively. An other compound Ba₃Fe₂Ta₆O₂₁ is partially ordered in respect to Ta⁵⁺ and Fe³⁺. Calculations of the Coulomb-part of lattice energy are discussed.