Über hexagonale Perowskite mit Kationenfehlstellen. XVI Die rhomboedrischen 12 L-Stapelvarianten Ba3AIIIM□O12 mit MV = Nb,Ta

On Hexagonal Perovskites with Cationic Vacancies. XVI. Rhombohedral 12 L-Stacking Polytypes Ba₃A^IIIM □O₁₂ with M^V = Nb, Ta The white quaternary oxides Ba₃LaM□O₁₂ with M^V = Nb, Ta belong to the group of hexagonal perovskites with cationic vacancies. They crystallize in a rhombohedral 12 L-structure (sequence (hhcc)₃; space group R3 m) with a = 5.75₁ Å; c = 28.1₁ Å (M^V = Nb); a = 5.74₆ Å; c = 28.2₀ Å (Ta) and Z = 3. Signs for the formation of isotypic compounds with A^III = Pr, Nd could be obtained as well.