Island-type structure

Crystal chemistry ofTa and Nb fluorides- Island type structure compounds... 

Island-type structures are inherent for compounds with X Me > 6. Typically X Me equals to 6, 7 or 8. We have found no documentation of niobium- or tantalum-containing compounds characterizing by X Me > 9. 

The radius of the second cation in known MuNbOFs, MU2Nb03F3 and Mul2Nb05F compounds containing bi- and trivalent metals, is usually similar to that of niobium s ionic radius. Such compounds cannot be considered as having an island-type structure and will be discussed later on. Only bismuth-containing compounds (Bi3+) display the presence of different cationic sublattices in their crystal structure. 

When 8 < X Me < 6 the structure remains an island-type structure with a respective increase in the coordination number. 

Vibration spectra of fluoride and oxyfuoride compounds correspond to X Me ratios, especially in the case of island-type structure compounds. Analysis of IR absorption spectra provides additional indication of the coordination number of the central atom. Fig. 45 shows the dependence on the X Me ratio of the most intensive IR bands, which correspond to asymmetric Me-F modes in fluoride complexes, as well as v(Me=0) and v(Me-F) in oxyfluoride complexes. Wave numbers of TaF5, NbF5 and NbOF3 IR spectra were taken from [283-286]. 

Oxyfluoride compounds that crystallize in island-type structures decompose and release light atoms into the gaseous phase ... 

Gibbs potential changes, 136 Interaction temperature with carbonates, 27,35 Island-type structure, 60-81... 

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