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Powder X-ray diffraction and cry0 electron diffraction techniques supported by computer simulations were used to determine the secondary structures of the series of heteropoly acid W A ) compounds Cs,%, P V M o 1~0 a . n H~0 (05x14). The structures of the caesium salts (25x14) are simple cubic with space group Pn3m. The free acid exhibits a tricliic lattice (P-1) for the majority of all crystals. A small fraction of thin crystals was detected with simple cubic structure (Pn3m). Computer simulations of the X-ray diffraction patterns suggested that the structure of Cs2H2PVh4011040~5H20 contains defects in the cation lattice. The caesium ion occupancy is U3. [H5O2lf-groups occupy 1/3 of the cation positions. The diffraction pattern of Cs4PVM01~0~5H20 could be simulated when only 3/4 of the anion positions are occupied. C~~H P V M O ,~O~.~H~O crystallises in a simple cubic lattice without any defects.The secondary structures of the investigated heteropoly salts can, therefore, be described by a single structure with variable site occupations, with the exception of Cs$€PVMolIOa~7H2O (caesium ion occupancy of 1 and polyanion occupancy of I). All salts belong to one phase with large stoichiometric variations. A mixture of acid and caesium salt, with the acid epitactly grown on the salts, an intergrowth-structure with varying caesium ion content, mixed crystals as well as the coexistence of a nanocrystalline X-ray-invisible form of free acid besides caesiurn salt could be excluded for the partly neuaalised materials. Selected physico-chemical properties of the HPA compounds, which depend on the caesium ion content, will be rationalised with the knowledge of the secondary structures.

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structure structure (solids and liquids) D 2000 29 -022 A New Unifying Structural Model of Heteropolymolybdate Salts: Microstructure and Thermal Stability of a Series of Molecular Oxides. -Heteropolymolybdate salts of composition CsxH 4−x PVMo 11 O 40 ·nH 2 O (0 ≤ x ≤ 4) precipitate from aqueous solutions of Cs 2 CO 3 and H 4 PVMo 11 O 40 . The Cs salts (2 ≤ x ≤ 4) have a simple cubic structure, space group Pn3m, as determined by powder XRD. The free acid exhibits a triclinic lattice (space group P-1) for the majority of all crystals and a simple cubic lattice (Pn3m) for a minor fraction of thin crystals. Computer simulation of the XRD pattern suggest that the structure of Cs 2 H 2 PVMo 11 O 40 ·5H 2 O contains defects in the cation lattice. -(BERNDT, S.; HEREIN, D.; ZEMLIN, F.; BECKMANN, E.; WEINBERG, G.; SCHUETZE, J.; MESTL, G.; SCHLOEGL, R.; Ber.