A pentagonal tunnel structure with copper in square planar coordination: The oxides KCuNb3O9 and KCuTa3O9

The structure of two new oxides KCuTa₃O₉ and KCuNb₃O₉ has been solved from X-ray powder data and by electron microscopy. Both compounds are orthorhombic, space group Pnc2 with $\text{a ? 8.8}$ Å, $\text{b ? 10.1}$ Å, and $\text{c ? 7.6}$ Å. Their host lattice is built up from corner-sharing MO₆ octahedra (M = Nb, Ta) forming pentagonal tunnels where the K⁺ ions are located. The copper ions are located in distorted perovskite CaCu₃Mn₄O₁₂-type cages and exhibit a square planar coordination. The relationships between these oxides and the TTB, HTB, ITB, and Ba_0.15WO₃ structures are discussed.