A new unifying structural model of heteropolymolybdate salts: Microstructure and thermal stability of a series of molecular oxides

Abstract

Abstract Powder X‐ray diffraction and cryo electron diffraction techniques supported by computer simulations were used to determine the secondary structures of the series of heteropoly acid (HPA) compounds Cs x H 4‐ x PVMo 11 O 40 · n H 2 O (0≤ x ≤4). The structures of the caesium salts (2≤ x ≤4) are simple cubic with space group Pn3m. The free acid exhibits a triclinic 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 Cs 2 H 2 PVMo 11 O 40 ·5H 2 O contains defects in the cation lattice. The caesium ion occupancy is 2/3. [H 5 O 2 ]+‐groups occupy 1/3 of the cation positions. The diffraction pattern of Cs 4 PVMo 11 O 40 ·5H 2 O could be simulated when only 3/4 of the anion positions are occupied. Cs 3 HPVMo 11 O 40 ·7H 2 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 3 HPVMo 11 O 40 ·7H 2 O (caesium ion occupancy of 1 and poly anion occupancy of 1). 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 witii varying caesium ion content, mixed crystals as well as the coexistence of a nanocrystalline X‐ray‐invisible form of free acid besides caesium salt could be excluded for the partly neutralised materials. Selected physico‐chemical properties of the HPA compounds, which depend on the caesium ion content, will be rationalised widi the knowledge of the secondary structures.