Niobium oxyfluorides

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

The data presented and discussed indicates that the mechanism of thermal decomposition of niobium oxyfluoride complex compounds is not trivial, but that some regularity can nonetheless be formulated regarding the processes, as follows ... 

Sodium reduction development directions, 336 diluted melts, 331-332 of K-Salt, 327-328 principals, 326 Solid-phase interaction mechanism, 34-37 niobium oxyfluorides, 26-31 tantalum oxyfluorides, 32-34 Solubility diagrams (NH4)5Nb3OF18, 22 K2NbF7 in HF solutions, 14 K2TaF7 in HF solutions, 14 RbsNbjOF,, 22-23 Solubility of peroxides, 307 Specific conductivity, 153, 164 Spontaneous polarization, 223 Structural characteristics for X Me=8, 61,... 

Gravimetric Methods.—The mixture of pentoxides of niobium and tantalum is redissolved in concentrated hydrofluoric acid and separated by Marignac s process (see p. 128) the potassium tantalum fluoride and potassium niobium oxyfluoride are then separately converted into the pentoxides as described above, and -weighed.1 This method has several disadvantages (1) The ratio of the solubilities of the two compounds on which the separation is based is only approximately 10 1, and the process is, therefore, necessarily inaccurate even when the recrystaflisation is repeated to a tedious extent the error approaches 1 per cent. (2) The concentration of the hydrofluoric add and of the potassium fluoride must be carefully controlled if the acidity is too low, an oxyfluoride of tantalum is precipitated if the acidity is too great, a normal fluoride of niobium is obtained. (3) Several platinum dishes are necessary. 

The hydrofluoric acid solution of niobic acid does not yield a precipitate on the addition of potassium fluoride (potassium niobium fluoride, K2NbF7, which is formed, being soluble in about 12-5 parts of water), whereas the hydrofluoric acid solution of tantalic add yields colourless, rhombic needles of potassium tantalum fluoride, K2TaF7 (which is soluble in about 150 parts of water under the same conditions), when treated with a saturated solution of potassium fluoride, carefully evaporated and cooled slowly. After removal of the tantalum, and with further concentration, any niobium present separates in pdates of potassium niobium oxyfluoride, K2Nb0F5.H20, if the hydrofluoric acid is not in excess, and in needles of potassium niobium fluoride, K2NbF7, if the hydrofluoric acid is in excess. 

Caesium Niobium Fluoride, CsF.NbFs or CsNbF6, is obtained in fine needles by repeated crystallisation of csesium niobium oxyfluoride, CSjNbOFj, from hydrofluoric acid.2 Another csesium niobium fluoride having the probable composition 7CsF.NbFs or Cs7NbF5 has been prepared by the action of a solution of caesium hydroxide in hydrofluoric add on niobic acid in the same solvent.3... 

Rubidium Niobium Fluoride, RbNbF6 or RbF.NbF5, is obtained by repeated crystallisation of rubidium niobium oxyfluoride, RbjNbOFg, from hydrofluoric acid.8 The double salt Rb2NbF7 or 2RbF.NbFs has also been reported.4... 

Copper Niobium Oxyfluoride, CuFa.Nb0F3.4H20 or CuNbOFg. 

Rvhidium Niobium Oxyfluoride, 2RbF.Nb0Fs or Rb bOFj or fNbQ5lRb2, is prepared similarly to the corresponding potassium... 

Thallimi Niobium Oxyfluoride, 2TlF.NbOF3 or Tl2NbOF6 or... 

Zinc Niobium Oxyfluoride, ZnF2.NbOF3.6HgO or ZnNbOFs.6HaO or J NbQSJZn.6H20, results from the action of zinc fluoride on a solution... 

It can be prepared by heating metallic niobium or any of the lower oxides, or the sulphide, carbide, or nitride of niobium, in air or oxygen. Ammonium niobium oxyfluoride, (NH4)jNbOFs, also yields the pentoxide on being heated in air. Solutions of the alkali niobates yield a white gel of the hydrated pentoxide when treated with mineral adds, sulphuric acid bring usually preferred. Insoluble niobates on fusion... 

When a solution of potassium niobium oxyfluoride, K2NbOFB, is boiled with potassium bicarbonate, a light, powdery, practically insoluble precipitate of the 1 3 salt, K20.3Nb20B.5H20, is thrown down.6... 

Niobium Peroxyfluorides or Fluoroxyperniobates.—Like niobic acid, the alkali niobium oxyfluorides have the property of taking up active oxygen by reaction with hydrogen peroxide. 

Rubidium Niobium Peroxyfluoride or rubidium fluoroxypemiobate, RbgNbOjFsHgO or 2RbF.NbOaFs.HaO, is obtained similarly, as thin, yellow plates, from rubidium niobium oxyfluoride, 2RbF.NbOFs.8... 

Powdered niobium metal, 20.0 g. (—200 mesh), and tin(II) fluoride, 52.0 g. (40 mesh),t are mixed in a molybdenum crucible in an Inconel- or nickel-pipe reactor approximately 3 in. in diameter and 10 in. long and heated to 400-500°C. in a stream of dry nitrogen. The niobium(V) fluoride volatilizes from the reaction mixture and condenses on the water-cooled lid of the reactor, which leaves metallic tin in the crucible. The yield of niobium(V) fluoride is 21.1 g., or 95% of theoretical. A very small amount of blue niobium oxyfluoride (composition of variable oxygen and fluorine content) often forms as an impurity because of the presence of minute amounts of oxygen. Anal. Calcd. for NbFs Nb, 49.44 F, 50.56. Found Nb, 49.43 F, 50.2. 

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