Oxyfluorides of Niobium

Two oxyfluorides of niobium having the compositions NbOF₃ and NbO₂F are known. The former has been obtained in small crystals by the action of hydrogen chloride on a fused mixture of niobium pentoxide and calcium fluoride at a red heat.1 The existence of the latter has been recognised only in double salts with potassium fluoride; it has not been isolated in the free state.

Double salts of niobium oxytrifluoride, NbOF₃, are obtained by the action of fluorides of the metals on solutions of niobium pentoxide in hydrofluoric acid, excess of the latter being avoided, otherwise the double fluorides are produced. These salts are much more stable than the double fluorides of niobium and other metals, and are, in fact, among the most stable of the pentavalent niobium compounds. The double oxyfluorides which are most readily prepared possess the general formula 2R^•F.NbOF₃.xH₂O, where R^• stands for a monovalent metal. Recrystallisation of double oxyfluorides having other compositions frequently yields salts of this type. These facts indicate the existence of a stable, divalent, complex anion of constitution [NbOF₅]'', the stability of which is confirmed by the great difficulty that is experienced in the electrolytic reduction of solutions of potassium niobium oxyfluoride, K₂NbOF₅.H₂O. It is of interest to note that double oxyfluorides of vanadium, tantalum, and molybdenum having similar compositions are also known. Double oxyfluorides of the alkali metals react with hydrogen peroxide to yield alkali niobium peroxyfluorides.

The following are known: NH₄F.NbOF₃ or NH₄NbOF₄; 5NH₄F.3NbOF₃.H₂O or (NH₄)₅.Nb₃O₃F₁₄H₂O; 2NH₄F.NbOF₃ or (NH₄)₂NbOF₅; 3NH₄F.NbOF₃ or (NH₄)₃NbOF₆. Of these, the compound 2NH₄F.NbOF₃ is the commonest, and is readily prepared by the action of ammonium fluoride on a solution of niobium pentoxide in hydrofluoric acid. It behaves similarly to the corresponding potassium salt, but is not isomorphous with it. It is isomorphous with ammonium tungsten oxyfluoride, 2NH₄F.WO₂F₂, and forms rhombic bipyramids in which a:b:c = 0.4184:1:1.0058. It can be heated to 170° C. without loss in weight. Its co-ordinated formula is .

By using excess of niobium pentoxide in the last preparation the compound 5NH₄F.3NbOF₃.H₂O is obtained. This is also very comparable in its behaviour to the corresponding potassium salt. The mother-liquor yields green crystals of composition NH₄F.NbOF₃ or on being evaporated. If excess of ammonium fluoride is employed, crystals of the compound 3NH₄F.NbOF₃ or are produced. All these double ammonium oxyfluorides can be heated to 100° C. without undergoing decomposition.

Cesium Niobium Oxyfluoride, 2Cs.F.NbOF₃ or Cs₂NbOF₅ or , is prepared similarly to the corresponding potassium salt (see below). It forms trigonal crystals.

4H₂O.Copper Niobium Oxyfluoride, CuF₂.NbOF₃.4H₂O or CuNbOF₅.4H₂O or is prepared by the action of copper fluoride on a solution of niobium pentoxide in hydrofluoric acid. It yields blue, monoclinic prisms which are very readily soluble in water and which are stable at 100° C. They are isomorphous with copper titanium fluoride, CuTiF₆.4H₂O, and with copper tungsten oxyfluoride, CuWO₂F₄.4H₂O.

The following are known: 4KF.3NbOF₃.2H₂O or K₄Nb₃O₃F₁₃.2H₂O; 5KF.3NbOF₃.H₂O or K₅Nb₃O₃F₁₄.H₂O; 2KF.NbOF₃.H₂O or K₂NbOF₅.H₂O; 3KF.NbOF₃ or K₃NbOF₆. Of these potassium compounds the salt 2KF.NbOF₃.H₂O or is the most important, and is, in fact, one of the commonest of niobium compounds, as it is the usual end product in the working of niobium minerals, and is the form in which niobium is separated from tantalum by the classical method due to Marignac. It is prepared by the addition of potassium fluoride to a hydrofluoric acid solution of niobium pentoxide, or by recrystallisation of other potassium niobium oxyfluorides or of potassium niobium fluoride, K₂NbF₇, from water. It undergoes recrystallisation unchanged in very thin leaflets from pure water. In the presence of a slight excess of hydrofluoric acid it forms thin, monoclinic scales which are isomorphous with potassium fluoroxytungstate, 2KF.WO₂F₂.H₂O, and with potassium titanium fluoride, 2KF.TiF₄.H₂O. Its crystal elements are: a:b:c = 0.992:1:0.980; β = 103°46'. It loses almost all its water at 100° C., but undergoes no further change and remains completely soluble in water when heated to 200° C. One part of the salt by weight dissolves in from 12.5 to 13 parts of water between 17° and 21° C. It is much more soluble in hot water and in water containing hydrogen fluoride or hydrogen peroxide, and is also much more soluble than the potassium tantalum oxyfluorides. The solution is precipitated by the addition of caustic soda or caustic potash; the precipitate is soluble in a slight excess of the alkali, but separates out again in crystalline form when large excess is added. In this respect the compound behaves differently from potassium titanium fluoride, but the reaction is not, however, available for the separation of niobium and titanium, as, when both metals are present, the precipitate also contains both. Hall and Smith have made a study of the differential action of seventy-four bases, both inorganic and organic, on solutions of potassium niobium oxyfluoride and potassium titanium fluoride.

The compound 5KF.3NbOF₃.H₂O or K₅Nb₃O₃F₁₄.H₂O is obtained from the mother-liquor in the preparation of the above salt when the potassium fluoride has been added only sparingly to the solution of niobic acid in hydrofluoric acid. The new mother-liquor thus obtained yields crystals of the composition 4KF.3NbOF₃.2H₂O or K₄Nb₃O₃F₁₃.2H₂O. The former of these gives, apparently, hexagonal prisms. They lose only part of their water at 100° C.; above this temperature hydrogen fluoride is evolved.

When the quantity of potassium fluoride added to the solution of niobic acid in hydrofluoric acid is somewhat greater than, that required for the formation of the salt 2KF.NbOF₃.H₂O, crystals of a salt having the composition 3KF.NbOF₃ or K₃NbOF₆ are obtained. This compound has also been prepared by fusing niobic acid with excess of potassium hydrogen fluoride. It is stable at 100° C. Its solution in hot water yields crystals of 2KF.NbOF₃.H₂O, so that the change

2KF.NbOF₃.H₂O + KF ⇔ 3KF.NbOF₃ + H₂O

appears to be reversible. A hydrofluoride of composition 3KF.NbOF₃. HF has been obtained from solutions containing large excess of hydrofluoric acid or of potassium fluoride. Slow crystallisation from concentrated solutions gives rise to very beautiful prismatic crystals, isomorphous with the double potassium tin fluoride, 3KF.SnF₄.HF; a:b:c = 0.6279:1:0.4900; β = 93°14'. This substance is also quite stable at 100° C., and loses hydrogen fluoride only on being very strongly heated.

If a solution of the compound 2KF.NbOF₃.H₂O is boiled for twelve hours while steam is introduced, a white, soft, micro-crystalline powder is obtained, the composition of which is 2KF.3NbO₂F or K₂Nb₃O₆F₅. A compound of the formula NbO₂F has not hitherto been isolated.

The compounds KF.Nb₂O₅.3H₂O and 3KF.2Nb₂O₅.5H₂O are of doubtful constitution. They are obtained as crystalline powders by fusing niobium pentoxide with potassium fluoride and extracting the melt with boiling water.

Rubidium Niobium Oxyfluoride, 2RbF.NbOF₃ or Rb₂NbOF₅ or , is prepared similarly to the corresponding potassium compound.

These are not so well defined as the potassium salts. Similar methods of preparation give rise to a precipitate of 2NaF.NbOF₃.2H₂O, which is, however, always associated with some of the salt NaF.NbOF₃.H₂O, and the two are difficult to separate. The compound 3NaF.NbOF₃ has, however, been obtained pure. A sodium-calcium-niobium oxyfluoride, NaCaNb₂O₆F, is also known. It is obtained by fusing lime, niobium pentoxide and sodium fluoride together. It forms colourless octahedra of density 4.196 to 4.341, and its refractive index for sodium light is 2.148 to 2.150; these figures agree with those given by natural pyrochlore.

Thallium Niobium Oxyfluoride, 2TlF.NbOF₃ or Tl₂NbOF₅ or , is obtained in colourless, transparent, rhombic crystals by the action in solution of thallium fluoride on niobium penta- fluoride.

6H₂O.Zinc Niobium Oxyfluoride, ZnF₂.NbOF₃.6H₂O or ZnNbOF₅.6H₂O or , results from the action of zinc fluoride on a solution of niobic acid in hydrofluoric acid. It gives rise to rhombic, hexagonal crystals, which are isomorphous with those given by zinc titanium fluoride, ZnF₂TiF₄.6H₂O.