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Niobates

The alkali niobates are most conveniently prepared by the action of caustic alkalis on niobic acid or on solutions of niobium oxytrifluoride. Other compounds of niobic acid and bases are generally prepared by fusing niobic acid with the oxide, hydroxide, carbonate, or other salt of the metal. Occasionally double decomposition of a soluble alkali niobate and a soluble salt of the metal has been employed. Larsson's method consists in precipitating a solution of potassium niobate with a salt of a metal; the dried precipitate is fused for thirty-six hours at a high temperature with boric acid, and the melt is boiled with water to which hydrochloric acid has been added. The residue consists of crystals of the insoluble niobate of the metal, usually the metaniobate.

Niobates obtained by any of these methods contain the basic oxide and acid oxide in proportions which vary from 1:4 to 5:1. The molecular complexities of most of them, as well as the complexity of the ions present in their solutions, are at present not understood; in many cases they are probably isomorphous mixtures of simpler salts. A revision of their composition is necessary. Many crystalline niobates are known, however, the composition of which appears to correspond to that of metaniobates, RNbO3 or R2O.Nb2O5, and hexaniobates, R8Nb6O19 or R2O.3Nb2O5. A few pyroniobates, R4Nb2O7 or 2R2O.Nb2O5, have also been prepared. The orthoniobates, R3NbO4 or 3R2O.Nb2O5, like the ortho vanadates, are very unstable and but little known, although their existence has been recognised in minerals.

Niobates are usually insoluble in water; even many of the alkali niobates are insoluble. Insoluble alkali niobates are formed when niobium pentoxide is fused with only small proportions of alkali carbonate. 7:6 Sodium niobate, 7Na2O.6Nb2O5.xH2O, is insoluble in solutions of high sodium-ion concentration, and is therefore precipitated by the addition of sodium salts to solutions of potassium niobate. Ammonium niobates are unknown; when excess of an ammonium salt is added to a solution of an alkali niobate, a voluminous precipitate is thrown down which is rapidly decomposed by water to yield niobicacid. This instability of ammonium salts is consistent with the weakness of niobic acid, which is also shown in the ease with which niobates are decomposed by water and acids to precipitate niobic acid. Arsenious, arsenic, oxalic, tartaric, citric and malonic acids do not produce precipitates with solutions of alkali niobates, probably because of the formation of soluble complex salts. Phosphoric, hydrocyanic and acetic acids yield precipitates with concentrated solutions of alkali niobates.

The following niobates are known, but the constitution of some of them is not beyond doubt:

Known Niobates

Aluminium Niobate

Aluminium Niobate, Al2O3.3Nb2O5.12H2O, is obtained as a white precipitate by the action of a solution of alum on sodium metaniobate, Na2O.Nb2O5.7H2O.

Barium Niobate

A salt having the composition 7BaO.6Nb2O5.18H2O is prepared by treating a hot solution of the corresponding sodium salt with dilute barium chloride solution.

Cadmium Metaniobate

The anhydrous salt, CdO.Nb2O5, is obtained by fusing potassium niobate with cadmium chloride. It forms yellowish-brown, glistening crystals, the density of which is 5.93. The hydrated salt CdO.Nb2O5.3½H2O is thrown down as a pale yellow precipitate on adding a solution of a cadmium salt to sodium metaniobate.

Cesium Niobates

Fusion of niobium pentoxide and caesium carbonate and extraction of the melt with water yields monoclinic crystals which have the composition 4Cs2O.3Nb2O5.14H2O, and which are isomorphous with the rubidium salt. Addition of alcohol to the aqueous solution furnishes crystals of 7Cs2O.6Nb2O5.30H2O.6

Calcium Metaniobate

Calcium Metaniobate, CaO.Nb2O5, is prepared by fusing niobium pentoxide with calcium fluoride in an excess of potassium fluoride; or the pyro-salt 2CaO.Nb2O5 is fused with boric acid and the melt extracted with water containing hydrochloric acid. It forms bright red, long, thin, apparently rhombic needles, which are doubly refracting. Density 4.12 to 4.48.

Calcium Pyroniobate

Calcium Pyroniobate, 2CaO.Nb2O5, is prepared by fusing with calcium chloride either niobium pentoxide or the precipitate obtained by the action of calcium chloride on potassium niobate solution; the melt is extracted with water containing hydrochloric acid, when the salt remains in small, colourless, glistening, prismatic crystals. Density at 17° C. 4.484.

Isomorphous mixtures of calcium niobates and sodium niobates have also been prepared. A calcium niobate occurs in the mineral pyrochlore.

Cobalt Metaniobate

Cobalt Metaniobate, CoO.Nb2O5, is prepared by heating together the two oxides at about 1100° C., or by fusing the precipitate thrown down by the action of cobalt nitrate on potassium niobate solution with boric acid. It is a dark blue, crystalline powder. Density 5.56.

Copper Metaniobate

The anhydrous meta-salt CuO.Nb2O5 consists of black, glistening crystals which are prepared similarly to the corresponding cobalt salt. Density 5.60. Addition of copper sulphate to an aqueous solution of sodium metaniobate, Na2O.Nb2O5.7H2O, yields a green precipitate of the hydrated compound CuO.Nb2O5.3½H2O. A dihydrate has been obtained by drying at 100° C.

Iron Niobates

Ferrous metaniobate, FeO.Nb2O5, is obtained more or less impure by fusing niobium pentoxide, ferrous fluoride, and excess of potassium chloride in a platinum crucible to a bright red heat. It forms long, steel-grey prisms. A ferric niobate, 2Fe2O3.3Nb2O5.8H2O, has been obtained by the action of ferric chloride on sodium metaniobate. Iron niobates enter into the composition of the natural niobites and tantalites.

Lithium Niobate

Lithium Niobate, 7Li2O.6Nb2O5.26H2O, is a crystalline substance which results from the action of lithium nitrate on a concentrated solution of potassium niobate, K2O.3Nb2O5.5H2O.

Magnesium Metaniobate (Anhydrous)

Magnesium Metaniobate (Anhydrous), MgO.Nb2O5, is prepared by fusing with boric acid the precipitate thrown down on the addition of magnesium chloride to a solution of potassium niobate. It yields short, prismatic, doubly refracting crystals. The heptahydrate, MgO.Nb2O5.7H2O, is obtained in white flakes by the addition of magnesium chloride to a solution of sodium metaniobate and drying on the water-bath. Drying at 100° C. yields a tetrahydrate.

If the precipitate thrown down by the addition of magnesium chloride to a solution of potassium niobate is fused with excess of magnesium chloride, or if niobium pentoxide is fused with excess of magnesium chloride under definite conditions, the salt 4MgO.Nb2O5 is obtained. This compound is remarkable for the high proportion of basic oxide present. It is alternatively described according to the method of preparation as consisting of (a) very small, white, hexagonal plates or prisms, of density 4.43; (b) of colourless or pale yellow, broad, transparent, hexagonal leaves belonging to the rhombic system, density 4.37. It is only slowly attacked by acids. This salt is sometimes accompanied by crystals of the orthoniobate, 3MgO.Nb2O5, and in one preparation magnesium pyrovanadate, 2MgO.Nb2O5, was produced. The latter consisted of small, steel grey, glistening prisms, which became white when brought to a red heat.

Manganese Metaniobate

Manganese Metaniobate, MnONb2O5, is prepared by fusing together niobium pentoxide, manganese fluoride and potassium chloride. It forms red, transparent, rhombic prisms, density 4.94. This salt is probably a constituent of the natural niobites. By fusing together niobium pentoxide, ferrous fluoride and manganese fluoride, Joly succeeded in preparing a substance which very closely resembled the natural niobites from Greenland. Analysis showed this substance to be an isomorphous mixture of ferrous metaniobate and manganese metaniobate, of the composition (2/3FeO.1/3MnO.)Nb2O5, which is approximately the composition of the niobites from Limoges.

Another manganese niobate, 3MnO.5Nb2O5, has been obtained by adding manganese sulphate to potassium niobate and fusing the precipitate with boric acid. It consists of small, greyish-yellow, prismatic rods, density 4.97.

Mercury Niobate

A salt having the probable composition Hg2O.Nb2O5.3H2O has been obtained by the action of mercurous nitrate on sodium niobate solution.

Potassium Niobates

Potassium Niobates are generally produced by fusion of the metal or the pentoxide with potassium hydroxide, potassium carbonate, or potassium nitrate, or by the action of solutions of caustic potash or of potassium carbonate on niobic acid. They are among the most stable of the niobates; their solutions can be boiled without precipitation of the acid. On being treated with sodium salts, for example sodium chloride, sodium niobates are precipitated.

When a solution of potassium niobium oxyfluoride, K2NbOF5, is boiled with potassium bicarbonate, a light, powdery, practically insoluble precipitate of the 1:3 salt, K2O.3Nb2O5.5H2O, is thrown down.

1:2 Potassium niobate, K2O.2Nb2O5.5½H2O, remains undissolved in the crystalline residue left after fusing equimolecular proportions of niobic acid and potassium carbonate and extracting the melt with water.

Thin, transparent plates of the anhydrous 3:4 salt, 3K2O.4Nb2O5, remain undissolved when niobic acid is fused with about twice its weight of potassium sulphate for several hours at a red heat and the melt is extracted with water.

Potassium metaniobate, KNbO3 or K2O.Nb2O5, is obtained as beautiful, straw-yellow, rectangular plates by fusing a mixture of equimolecular proportions of potassium carbonate and niobic acid; in this case the niobic acid is previously fused with calcium fluoride. The crystals are extracted from the product with boiling, dilute sulphuric acid.

8:7 Potassium niobate, 8K2O.7Nb2O5.32H2O, precipitates out on slow evaporation of solutions of either the 4:3 salt or the 7:6 salt. It forms rhombic bipyramids which can be recrystallised unchanged; a:b:c = 0.9584:1:0.7083. Twenty-three molecules of water are lost at 100° C. It readily yields supersaturated solutions. When its aqueous solutions are treated with a current of carbon dioxide they precipitate salts which contain a larger proportion of niobic acid.

7:6 Potassium niobate, 7K2O.6Nb2O5.27H2O, is precipitated by the addition of alcohol to an aqueous solution of the 4:3 salt. The product is redissolved in water and reprecipitated several times.

4:3 Potassium niobate, 4K2O.3Nb2O5.16H2O, is the commonest of the potassium niobates. It forms large monoclinic prisms which are isomorphous with 4:3 potassium tantalate, 4K2O.3Ta2O5.16H2O. Niobic acid is fused with from two to three times its weight of potassium carbonate in the blowpipe until carbon dioxide is no longer evolved; the melt is extracted with water and the extract is either evaporated in vacuo or allowed to evaporate spontaneously. The crystals effloresce in the air and lose twelve molecules of water of crystallisation at 100° C. At a red heat the salt becomes yellow but does not melt; after being so heated it is only partially soluble in water.

3:2 Potassium niobate, 3K2O.2Nb2O5.13H2O, is obtained in rhombic crystals by adding excess of potassium hydroxide to a solution of the 4:3 salt and evaporating slowly. It effloresces rapidly in the air and loses seven molecules of water of crystallisation at 100° C.

2:1 Potassium niobate or potassium pyroniobate, 2K2O.Nb2O5.11H2O, is an insoluble powder which is obtained by fusing the 1:2 salt with excess of potassium carbonate and extracting the product with water.

5:2 Potassium niobate and 3:1 potassium niobate (the ortho-salt) have also been reported.

Rubidium Niobates

When niobium pentoxide is fused with rubidium carbonate and the melt is extracted with water, fine, silky needles of composition 3Rb2O.4Nb2O5.9½H2O are left behind. Concentration of the filtrate yields monoclinic crystals of 4Rb2O.3Nb2O5. 14H2O, which are isomorphous with the corresponding tantalum salt and with the corresponding caesium niobate and tantalate. a:b:c = 0.8815:1:1.0491; β = 95°53'. It slowly loses some of its water on exposure to air.

Silver Metaniobate

Silver Metaniobate, Ag2O.Nb2O5.2H2O, is obtained as a white precipitate by the addition of silver nitrate to an aqueous solution of sodium metaniobate. It becomes pale yellow on being dried and gradually darkens in sunlight. Addition of dilute silver nitrate solution to a solution of 7Na2O.6Nb2O5 precipitates the corresponding silver salt, 7Ag2O.6Nb2O5.5H2O, which is a white, insoluble substance.

Sodium Niobates

Sodium Niobates are comparable to the potassium niobates. They differ in that they can be precipitated from solutions of potassium niobates by the action of neutral sodium salts. The usual method of preparation consists in fusing niobium pentoxide with caustic soda in varying proportions and, after washing away excess of caustic soda, crystallising the product from water. It is to be noted, however, that this method, in the hands of different investigators, has yielded several sodium niobates which differ in the relative amounts of base and acid present. It appears that the only sodium niobates hitherto prepared which can be definitely regarded as chemical individuals are the meta- salt, Na2O.Nb2O5.7H2O, and the 7:6 salt, 7Na2O.6Nb2O5.31H2O or 32H2O. The commonest potassium niobate is the 4:3 salt; 4:3 sodium niobate has not been prepared.

2:3 Sodium niobate, 2Na2O.3Nb2O5.9H2O, is said to have been prepared in small crystals by fusing niobium pentoxide with caustic soda and extracting the melt with boiling water, but Bedford was unable to obtain the compound by this process.

Sodium Metaniobate

Sodium Metaniobate, Na2ChNb2O5 or NaNbO3. - Anhydrous sodium metaniobate is obtained in white, strongly refractive, cubic crystals by ignition of the 7:6 salt, or by fusing equimolecular proportions of niobic acid and sodium carbonate in a flux of sodium fluoride. Excess of sodium fluoride is removed from the melt with water, in which the niobate crystals are insoluble. Density, 4.512 to 4.559. Hydrated sodium metaniobate, Na2O.Nb2O5.6H2O or 7H2O, is contained in the residue obtained after a fused mixture of niobium pentoxide and caustic soda has been extracted with a small volume of cold water, or in the residue obtained after boiling niobium pentoxide with caustic soda solution. Extraction of these residues with hot water yields small, glistening, triclinic crystals, which have also been obtained by the slow, spontaneous evaporation of the mother-liquors from the 7:6 salt. a:b:c = 0.9559:1:0.8394; α = 71°20', β = l05°30', γ = 54°7'. One gram of the hydrated salt dissolves in 75 cc. of water at 100° C. and in 200 cc. of water at 14° C. Aqueous solutions slowly become cloudy on standing in the air, probably through the action of carbon dioxide, which precipitates the 1:4 salt, Na2O.4Nb2O5.H2O. Addition of alcohol to the aqueous solution yields the 7:6 salt,

7(Na2O.Nb2O5) = 7Na2O.6Nb2O5 + Nb2O5.

Niobic acid, or more probably acid niobates, remain in solution. On the other hand, by passing a stream of carbon dioxide through an aqueous solution of the 7:6 salt, the 1:1 salt or metaniobate is precipitated; these two sodium niobates hence are mutually convertible.

7:6 Sodium niobate, 7Na2O.6Nb2O5.31H2O or 32H2O or 36H2O, is prepared (a) by adding a large excess of caustic soda to a solution of potassium niobium oxyfluoride, K2NbOF5, washing the heavy precipitate of sodium niobate with cold water and crystallising the product from boiling water; (b) by fusing niobium pentoxide with about four times its weight of caustic soda or sodium carbonate, washing the melt with cold water to remove excess of caustic soda, and crystallising the product from boiling water; (c) by fusing niobium pentoxide with potassium carbonate in a gold or platinum crucible and precipitating the aqueous extract with sodium chloride solution. A micro-crystalline powder is usually obtained, but the salt crystallises in prisms which can be dried in the air without undergoing decomposition. It is readily and completely soluble in water, the aqueous solutions appearing to contain the undecomposed salt, because on the addition of salts of barium, silver or zinc, the corresponding 7:6 niobates of these metals are precipitated. Treatment of the aqueous solution with carbon dioxide, however, precipitates the meta-salt, which is also obtained by spontaneous evaporation of the mother-liquors from which the 7:6 salt has been crystallised.

3:4, 8:7, 6:5, 3:2, and 3:1 sodium niobates have also been reported, but their identities are doubtful.

A double sodium ammonium niobate of probable formula 5(NH4)2O. Na2O.24Nb2O5.30H2O or [5/6(NH4)2.1/6Na2]O.4Nb2O5.5H2O has been prepared by the addition of ammonium sulphate or ammonium chloride to a solution of sodium niobate. Similarly, addition of caustic potash which contains some caustic soda to a solution of potassium niobate has yielded a sodium potassium niobate, Na2O.3K2O.3Nb2O5.9H2O.

Thorium Niobate

Thorium Niobate, 5ThO2.16Nb2O5, is prepared by adding thorium sulphate to a solution of potassium niobate, fusing the precipitate with borax and boiling the melt with water containing hydrochloric acid. It forms doubly refracting, prismatic needles; density 5.21.

Yttrium Niobates

Larsson's method yields very small needles of the composition Y2O3.3Nb2O5; density 4.83. The metaniobate, Y2O3.Nb2O5, is obtained by fusing a mixture of niobium pentoxide and yttrium chloride in excess of potassium chloride at a white heat; alternatively, the precipitate obtained by the addition of yttrium chloride to a solution of potassium niobate is fused with anhydrous yttrium chloride. It forms doubly refracting, white octahedra; density 5.52. This compound is of interest in that it is probably a constituent of the natural yttrotantalites and fergusonites.

Zinc Metaniobate

Zinc Metaniobate, ZnO.Nb2O5, is prepared by Larsson's method. It forms brown, apparently rhombic aggregates; density 5.69. Addition of zinc sulphate to a solution of the 7:6 sodium salt yields a white, insoluble precipitate of 7ZnO.6Nb2O5.25H2O.

Zirconium Niobate

The salt ZrO2.5Nb2O5 has been obtained in cloudy, rod-like needles, similarly to the thorium salt; density 5.14. This salt is remarkable for its large content of acid oxide.
In addition to the foregoing, more or less impure niobates of beryllium, cerium, lanthanum and nickel have been prepared.

Niobic acid also reacts with hydroxylamine. When the 4:3 potassium niobate is digested for several days with concentrated ammonium hydroxide and hydroxylamine hydrochloride, a precipitate is obtained which, after being washed and dried, has the composition 3NH2OH.HNbO3. The same process frequently yields the compound 5NH2OH.2HNbO3.H2O. These are both sparingly soluble, white substances, which decompose explosively on being heated. In contact with water at ordinary temperatures some hydroxylamine and a little niobic acid pass into solution. Analogous compounds with hydroxylamine are also given by vanadic acid, phosphoric acid, tungstic acid, etc.

The double salts that are formed from niobium oxytrifluoride, NbOF3, and niobium oxytrichloride, NbOCl3, with the halides of the metals can in some cases also be looked upon as salts of niobium pentoxide. Sodium niobium oxyfluoride, 2NaF.NbOF3. 2H2O, can be written Nb2O5.4NaF.6HF.H2O. These double fluorides are hence alternatively styled fluoroxyniobates. This name is not to be preferred, because the ions which are formed in the solutions appear to be of the type [NbOF3.nF]n'. The above-mentioned sodium salt, for instance, most probably yields the complex ion [NbOF5]''.

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