Crystal structures of some niobium and tantalum oxides. VII. K4+5xTa16−xO42 (x ≈ 0.3)

K_4+5xTa_16?xO₄₂ 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 $\text{P6}{}_3\text{mcm}\text{, Z = 1}$. 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 K₂O:Ta₂O₅ and Rb₂O:Nb₂O₅ systems. Its significance in this respect is discussed.