Zur Kristallchemie der Oxometallate des Hafniums

On the Crystal Chemistry of Oxometallates of Hafnium The contribution shows Hf⁴⁺ in oxides and oxometallates with typical and untypical coordination by oxygen. First of all the HfO6‐octahedra exist besides the Oxides of Hafnium in different Hf‐molybdates and ‐vanadates. A special feature is the formation of complex surroundings of to each other isolated HfO₆‐octahedra, forming HfO₃(Mo₆O₂₁)]⁸⁻‐, HfO₃(Mo₁₂O₃₉)]⁸⁻‐ and HfO₃(Rb₂O₁₅)]³⁰⁻‐groups. HfO₆‐octahedra are incorporated in polyhedra chains too. Interesting points of view of the crystal chemistry of Hf‐oxometallates are Hf/O‐polyhedra showing coordination numbers C.N. > 6. For example two times capped trigonal prisms (hp/ht‐HfMo₂O₈) and three times capped trigonal prisms ht/hp‐HfO₂ (Cottunnit‐Type) are representative polyhedra with C.N. = 8 and C.N. = 9. In the fluorite related compounds (Ca₂Hf₇O₁₆) Hf⁴⁺ is situated inside a damaged cube (one corner is missing). At least hexagonal HfO₁₂‐antiprisms (C.N. = 12) had been found in one of the trigonal form of HfMo₂O₈. In the same crystal structure it shows (C.N. = 6) trigonal HfO₆‐Prismens. Hf⁴⁺ has in addition no problems to accept statistical distributions with other elements. It adapts crystal structures of pyrochlores and perowskites of different distortion. Finally one compound, nearly free of oxygen (Hf₉Mo₄NiO_0,84) is reported, it shows interesting metal clusters.