Synthesis and Crystal and Electronic Structures of the Na<sub>2</sub>(Sc<sub>4</sub>Nb<sub>2</sub>)(Nb<sub>6</sub>O<sub>12</sub>)<sub>3</sub> Octahedral Niobium Cluster Oxide. Structural Correlations between A<i><sub>n</sub></i>BM<sub>6</sub>L<sub>12</sub>(Z) Series and Chevrel Phases
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We report here the synthesis and crystal and electronic structures of the Na2(Sc4Nb2)(Nb6O12)3 niobium oxide whose structure is related to that of Ti2Nb6O12. It constitutes a new member of the larger AnBM6L12(Z) families (A = monovalent cation located in tetrahedral cavities of units, B = monovalent or trivalent cations located in octahedral cavities of units, M = rare earth, Zr, or Nb, Z = interstitial except for M = Nb). The structural relationships between the AnBM6L12(Z) series (M6Li12La6 unit-based compounds with a M6Li6Li-a6/2La-i6/2 cluster framework) and Chevrel Phases (M6Li8La6 unit-based compounds with a M6Li2Li-a6/2La-i6/2 cluster framework) are shown in terms of M6L18 and M6L14 unit packing. Despite a topology similar to that encountered in Chevrel Phases, intercalation properties are not expected in the Nb6Oi6Oi-a6/2Oa-i6/2 cluster framework-based compounds. Finally, it is shown, from theoretical LMTO calculations, that a semiconducting behavior is expected for a maximum VEC of 14 in the Nb6Oi6Oi-a6/2Oa-i6/2 cluster framework.
