Thermal decomposition of oxyfluoroniobates

The thermal decomposition of alkali metal oxyfluoroniobates is also not a trivial process. MNbOF4 compounds (where M = alkali metal) with a chain-type structure are relatively stable up to temperatures in the range of 500-600°C. Fig. 90 presents mass loss dependences on temperature of several MNbOF4 compounds. As can be seen, among the compounds presented, only CsNbOF4 exhibits significantly different behavior, beginning its thermal decomposition at a lower temperature of about 400°C. 

The phase composition of products obtained from the thermal treatment of LiNbOF4 and NaNbOF4 was investigated using X-ray diffraction and vibration spectroscopy, as reported in [379]. Compounds with the following structures were found M2NbOF5, MNb02F2 and MNbC 3, where M = Li or Na. 

The formation of M2NbOF5 can be expressed by the following decomposition mechanism  

The formation of MNb02F2 serves as a source for the following decomposition process, which yields metaniobate of the alkali metal, MNb03  

The MNbOF4 formed in the interaction described by Equation (92) then participates again in the decomposition processes described by Equations (90) and (91). 

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Thermal decomposition of oxyfluoroniobates yields two main compounds containing higher and lower oxygen concentrations relative to the initial compound ... 

Tantalum powder particle size, 334 production, 332 Thermal decomposition of CoNbOFj, 54,210 fluorotantalates, 195, 200 oxyfluoroniobates, 202-205,210 Nb02F, 25,210-211 niobium hydroxides, 300-303 niobium peroxide, 305-308 tantalum hydroxide, 300-303 tantalum peroxide, 305-308 Tributyl phosphate, 279-281... 

The partially reduced form of niobium accounts for the color change of samples that underwent thermal treatment in vacuum or inert atmospheres. Whereas the thermal treatment of the mixture in air leads to the simultaneous oxidation of Nb4+ by oxygen, this is actually equivalent to the replacement of fluorine ions by oxygen ions in the complex structure of oxyfluoroniobate. Extended thermal treatment of systems containing LiNbOF4 and LiF yields a mixture of LiF and LiNbOs as the final thermal decomposition product. 

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