Crystal structure of TlTeF5

The crystal structure of a previously unknown polymorph of TlTeF5 has been solved and refined to R1 = 0.045 and ωR2 = 0.088 on the basis of 791 independent reflections recorded from a single crystal with an automatic four-circle diffractometer. This new form crystallises with orthorhombic symmetry (space group Pnma) and unit cell parameters: a = 1682.3 (2) pm, b = 622.2 (1) pm, c = 479.5 (1) pm, Z = 4. The TeIV atoms are fivefold coordinated and their lone pairs E are stereochemically active. The Tl atoms are 12-fold co-ordinated, at the centre of anticuboctahedra. The independent TeF5E polyhedra and the Tl atoms are distributed at the corners of a highly distorted centred cubic network and constitute (NH4)2SiF6-like sheets parallel to (100) and separated by the lone pairs E. This structure can also be described as an alternate stacking along the b axis of purely anionic square 44 layers F4 and of mixed TlTeFE layers. Comparisons are made with other MITeF5 pentafluorotellurates.