A NEW UNIFYING STRUCTURAL MODEL OF HETEROPOLYMOLYBDATE SALTS - MICROSTRUCTURE AND THERMAL-STABILITY OF A SERIES OF MOLECULAR OXIDES

Powder X-ray diffraction and cryo electron diffraction techniques supp orted by computer simulations were used to determine the secondary str uctures of the series of heteropoly acid (HPA) compounds CsxH4-x PVMo1 1O40. nH(2)O (0 less than or equal to x less than or equal to 4). The structures of the caesium salts (2 less than or equal to x less than o r equal to 4) are simple cubic with space group Pn3m. The free acid ex hibits a triclinic lattice (P-l) for the majority of all crystals. A s mall fraction of thin crystals was detected with simple cubic structur e (Pn3m). Computer simulations of the X-ray diffraction patterns sugge sted that the structure of CS2H2PVMO11O40. 5H(2)O contains defects in the cation lattice. The caesium ion occupancy is 2/3. [H5O2](+) -group s occupy 1/3 of the cation positions. The diffraction pattern of Cs4PV Mo11O40. 5H(2)O could be simulated when only 3/4 of the anion position s are occupied. Cs3HPVMo11O40. 7H(2)O crystallises in a simple cubic l attice without any defects. The secondary structures of the investigat ed heteropoly salts can, therefore, be described by a single structure with variable site occupations, with the exception of Cs3HPVMo11O40. 7H(2)O (caesium ion occupancy of 1 and polyanion occupancy of 1). All salts belong to one phase with large stoichiometric variations. A mixt ure of acid and caesium salt, with the acid epitactly grown on the sal ts, an intergrowth-structure with varying caesium ion content, mixed c rystals as well as the coexistence of a nanocrystalline X-ray-invisibl e form of free acid besides caesium salt could be excluded for the par tly neutralised materials. Selected physico-chemical properties of the HPA compounds, which depend on the caesium ion content, will be ratio nalised with the knowledge of the secondary structures.