KTi6Nb5O25 : A member of a series of chemically twinned rutile oxides (AM3O9) (M2O4)n

A new oxide KTi₆Nb₅O₂₅ with a structure related to that of KTi₂Ta₅O₁₅ (1) has been prepared and described. It crystallizes in the orthorhombic system with unit-cell dimensions $\text{a}\text{= 6.611(4)}\text{A}\text{?}$, $\text{b}\text{= 8.880(8)}\text{A}\text{?}$, $\text{c}\text{= 30.154(18)}\text{A}\text{?}$ and space group Cmcm. The tunnel structure of this oxide has been studied from powder data. It is built up from rutile slabs whose width is determined by strings of six octahedra. This compound may be described as the fourth member of a chemically twinned rutile series (AM₃O₉) (M₂O₄)_n, KTi₂Ta₅O₁₇ being the second member of the series.