Niobium oxygen compounds

Since niobates and tantalates belong to the octahedral ferroelectric family, fluorine-oxygen substitution has a particular importance in managing ferroelectric properties. Thus, the variation in the Curie temperature of such compounds with the fluorine-oxygen substitution rate depends strongly on the crystalline network, the ferroelectric type and the mutual orientation of the spontaneous polarization vector, metal displacement direction and covalent bond orientation [47]. Hence, complex tantalum and niobium fluoride compounds seem to have potential also as new materials for modem electronic and optical applications. 

Alongside this, IR spectra of M3NbOF6 type compounds display a strong absorption band at 920 cm"1 indicating the presence of a double niobium-oxygen bond, Nb=0 [57, 115, 155, 156]. Kaidalova [156] explained this contradiction as a phenomenon that is related to the hampered rotation of the NbOF63 polyhedron in the equatorial plane. 

The steric similarity of oxygen and fluorine ions enables the formation of coordination-type structures in some tantalum and niobium oxyfluoride compounds. 

One of the most important parameters that defines the structure and stability of inorganic crystals is their stoichiometry - the quantitative relationship between the anions and the cations [134]. Oxygen and fluorine ions, O2 and F, have very similar ionic radii of 1.36 and 1.33 A, respectively. The steric similarity enables isomorphic substitution of oxygen and fluorine ions in the anionic sub-lattice as well as the combination of complex fluoride, oxyfluoride and some oxide compounds in the same system. On the other hand, tantalum or niobium, which are the central atoms in the fluoride and oxyfluoride complexes, have identical ionic radii equal to 0.66 A. Several other cations of transition metals are also sterically similar or even identical to tantalum and niobium, which allows for certain isomorphic substitutions in the cation sublattice. 

The ratio between the anionic and cationic radii leads to coordination numbers, the lowest of which is 6, which correspond to a octahedral polyhedron of anions around a central cation [135]. In this case, the compound structure type depends on the ratio of total number of anions and cations. The total number of anions (X) is calculated by summing up the number of oxygen (O) ions and of fluorine (F) ions X=0+F, while the total number of cations (Me) is the number of tantalum ions, niobium ions and other similar cations. 

The fluorination process aims to decompose the material and convert tantalum and niobium oxides into complex fluoride compounds to be dissolved in aqueous solutions. The correct and successful performance of the decomposition process requires a clear understanding of the oxygen-fluorine substitution mechanism of the interaction itself. 

Thermal treatment of the compounds obtained from the hydrolysis leads to their decomposition, yielding tantalum or niobium oxides and gaseous oxygen. The processes of thermal decomposition are given as follows [512] ... 

At temperatures above 45°C, however, (NRj MOs compounds decompose yielding gaseous ammonia, NH3, and oxygen as well as tantalum or niobium oxides ... 

Niobium in its +5 oxidation state forms both oxygen and halogen compounds (Niobium in oxidation states of+2, +3 and +4 also forms compounds—for example, niobium(ll) dioxide and niobium(IV) tetraoxide) ... 

The lithium compound was heated at about 520 K and niobium pentamethoxide at 470 K in a stream of argon containing oxygen. Lithium niobate was deposited on the reaction vessel which was heated to 720 K. 

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