Ionenaustauschexperimente an ternären Tantalaten und Niobaten mittels Halogenidschmelzen — ein Zugang zu neuen Bleitantalaten

Exchange Reactions of Ternary Tantalates and Niobates with Halide Melts — a Way to New Lead Tantalates Ternary tantalates and niobates of the hexagonal structure-type A_(n+1)/m^m+M_3n+1O_8n+3 (A^m+ = Na⁺, Cu⁺, Ag⁺,…; M = Ta, Nb) were heated with surplus CuCl, PbCl₂, LaCl₃ or BiCl₃ (for example T = 700°C in case of PbCl₂). In some of these cases it was possible to exchange the A^m+-ions for the cations of the halide melts by maintaining the basic structure (for example: Cu₃Ta₇O₁₉ + BiCl₃ = BiTa₇O₁₉ + 3 CuCl). In different reactions the stack along the crystallographic c-axis and furthermore the proportion O/M (for example: 7 Cu₅Ta₁₁O₃₀ + 35/2 PbCl₂ = 11 Pb_1,5Ta₇O₁₉ + PbO + 35 CuCl) changed. Nevertheless, it was not possible to substitute cations of the halide melt for A^m+-ions in every direction (not possible for example: LaTa₇O₁₉ + BiCl₃). When CuCl? and PbCl₂-melt were added to Ag₂Ta₄O₁₁ and Na₂Nb₄O₁₁, these niobates showed exange reactions by changing their structure completely. If BiCl₃-melt was added, these niobates decayed while forming B? Nb₂O₅. The new hexagonal tantalates Pb_1,5Ta₇O₁₉ (a = 6,2411(7) Å; c = 19,977(3) Å) and PbTa₄O₁₁ (low-temperature modification; a = 6,2364(3) Å; c = 36,851(3) Å) were the first representatives of the structure-type A_(n+1)/m^m+M_3n+1O_8n+3 with A^m+ = Pb²⁺, which were found in a reaction with PbCl₂ (Cu₃Ta₇O₁₉ + 3/2 PbCl₂ = Pb_1,5Ta₇O₁₉ + 3 CuCl; Ag₂Ta₄O₁₁ + PbCl₂ = PbTa₄O₁₁ + 2 AgCl). These two compounds, which are probably metastable, could only be achieved by exange reactions of ions.