Crystal structures of some niobium and tantalum oxides. VII. K 4+5 xTa 16- xO 42 ( x ≈ 0.3)

K _{4+5 x}Ta _{16- x}O ₄₂ crystallizes in the hexagonal system with unit-cell dimensions (from single-crystal data) a = 9.085(6), c = 12.254(8) Å and space group {P6 ₃}/{mcm}, Z = 1 . The structure was solved by Patterson and Fourier techniques and refinement by full-matrix least-squares methods using 271 reflections, measured by counter techniques, with I ⩾ 3.5 σ ( I), resulted in an R of 0.060 ( R_w = 0.051). The structure consists of units of six octahedra, edge- and corner-shared to one another, that are linked by corner sharing through a single octahedron. This structure provided the "key" to other structures in the K ₂O:Ta ₂O ₅ and Rb ₂O:Nb ₂O ₅ systems. Its significance in this respect is discussed.