CRYSTAL-STRUCTURES OF SOME NIOBIUM AND TANTALUM OXIDES .7. K4+5XTA16-XO42 (X ALMOST-EQUAL-TO 0.3)

K4+5xTa16-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 P63 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 (Rw = 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. © 1979."