THE CRYSTAL-STRUCTURES OF K8TA6O19 CENTER-DOT 16H(2)O AND K7NATA6O19-CENTER-DOT-14H(2)O

By alkaline digestion of Ta2O5 with p.a. KOH transparent single crysta ls of the composition K8Ta6O19 . 16 H2O are formed. When technical gra de KOH is used, the same kind of synthesis yields crystals of the comp osition K7NaTa6O19 . 14H(2)O. The latter compound has been given the f ormula K8Ta6O19 . 14 H2O until now In both cases the isopolyoxoanion [ Ta6O19](8-) consists of six TaO6-octahedra connected by edge sharing. This means that the heavy atom partial structure found by Lindquist et al. is confirmed. Additionally the complete structures including the atomic positions of the oxygen atoms of the polyanions as well as thos e of the cations and crystal water molecules (without hydrogen positio ns) are determined.