Niobium niobate

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Lithium Niobate. Lithium niobate [12031 -64-9], LiNbO, is normally formed by reaction of lithium hydroxide and niobium oxide. The salt has important uses in switches for optical fiber communication systems and is the material of choice in many electrooptic appHcations including waveguide modulators and sound acoustic wave devices. Crystals of lithium niobate ate usually grown by the Czochralski method foUowed by infiltration of wafers by metal vapor to adjust the index of refraction. 

Occurrence. Niobium and tantalum usually occur together. Niobium never occurs as the metal, ie, ia the free state. Sometimes it occurs as a hydroxide, siUcate, or borate most often it is combiaed with oxygen and another metal, forming a niobate or tantalate ia which the niobium and tantalum isomorphously replace one another with Htde change ia physical properties except density. Ore concentrations of niobium usually occur as carbonatites and are associated with tantalum ia pegmatites and alluvial deposits. Principal niobium-beariag minerals can be divided iato two groups, the titano- and tantalo-niobates. 

Niobic Acid. Niobic acid, Nb20 XH2O, includes all hydrated forms of niobium pentoxide, where the degree of hydration depends on the method of preparation, age, etc. It is a white insoluble precipitate formed by acid hydrolysis of niobates that are prepared by alkaH pyrosulfate, carbonate, or hydroxide fusion base hydrolysis of niobium fluoride solutions or aqueous hydrolysis of chlorides or bromides. When it is formed in the presence of tannin, a volurninous red complex forms. Freshly precipitated niobic acid usually is coUoidal and is peptized by water washing, thus it is difficult to free from traces of electrolyte. Its properties vary with age and reactivity is noticeably diminished on standing for even a few days. It is soluble in concentrated hydrochloric and sulfuric acids but is reprecipitated on dilution and boiling and can be complexed when it is freshly made with oxaHc or tartaric acid. It is soluble in hydrofluoric acid of any concentration. 

Sodium niobate(7 6) [12201 -59-17, 7Na20 6Nb20 3IH2O or Naj4Nbj2 037-31H2 0, forms a crystalline precipitate when a hot solution of a soluble niobium compound is added to a hot concentrated sodium hydroxide solution. It is insoluble in the presence of excess sodium hydroxide but is sparingly soluble in pure water. It also can be formed by addition of sodium hydroxide or chloride to a solution of the 4 3 potassium niobate. 

Lithium niobate [12031 -63-9] Nb20 or LiNbO, is prepared by the soHd-state reaction of lithium carbonate with niobium pentoxide. After... 

Niob, Niobium, n. niobium (columbium). Niobat, n. niobate (columbate). 

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. 

Other examples of compounds that are considered MeX3-type compounds are lithium hexafluorotantalate (-niobate), LiTaF6 and LiTaF6, which have crystal structures similar to A1F3. Both lithium and tantalum (niobium) cations have similar ionic radii and are located in the centers of octahedrons that are composed of fluorine anions. 

Decomposition of lithium niobate, LiNb03, by molten ammonium hydrofluoride can be performed even at temperatures as low as 130-260°C [122]. The process also enables the separation of niobium and lithium, yielding ammonium oxyfluoroniobate and lithium fluoride. The interaction can be represented by Equation (132) ... 

Agulyanskaya et al. [507] investigated the impact of fluorine content on the particle size of niobium and tantalum oxides and powdered lithium niobate and tantalate prepared from the oxides. It was shown that fluorine concentrations lower than 10"2% wt. do not influence particle size and result in a set minimum particle size. This concentration range was referred to as being non-... 

Ninhydrin, 22 101 Ninhydrin-color reaction amino acids, 2 570 Niobates, 27 152-153 24 315 Niobia-phosphate catalytic aerogels, 2 763t Niobic acid, 27 152 Niobic salts, 27 152-153 Niobium (Nb), 27.132-157 24 313, 315. See also Nb-Ti entries Niobium compounds Niobium metal analytical methods for, 27 142-144 dissolution methods for, 27 142 economic aspects of, 27 140-142 effect on stainless steel corrosion resistance, 7 809... 

The results of studies of niobates, tantalates, and mixed oxide compounds containing niobium(v) or tantalum(v) are summarized in Table 6. 

T able 6 Niobates, tan talates and mixed oxide compounds containing niobium (v)... 

Gmelin Handbook of Inorganic Chemistry, System No. 49 Niobium, Part B, Section 3 Oxoniobates (excl. Alkali Niobates), 8th edn., Gmelin Inst. 1971. 

Niobium is always covered with an oxide layer. In aqueous solutions only some niobium compounds are soluble, mostly in the form niobate anions, for example, [H ,Nb60i9]( -- >-. 

Tantalum can be separated from niobium by recrystallization of the double potassium fluorides. In the commercial process the ore is fused with caustic soda. The insoluble sodium niobate, sodium tantalate, and... 

Lithium niobate is a ferroelectric material used as an optical switch. Preparation by the simple ceramic method leads to problems in obtaining the correct stoichiometry, and a mixture of phases often results. Several sol-gel preparations have been described, their advantage being the lower temperature required for the preparation and the greater homogeneity of the product. One such preparation starts with lithium ethoxide (LiOC2H5 (or LiOEt)) and niobium ethoxide Nb2(OEt)io. Each ethoxide was dissolved in absolute ethanol and the two solutions mixed. The addition of water leads to partial hydrolysis giving hydroxy-alkoxides, for example ... 

The sol-gel method of preparing lithium niobate used lithium and niobium alkoxides. Alkoxides are often used in CVD methods, but unfortunately for the preparation of lithium niobate, lithium alkoxides are much less volatile than niobium alkoxides and to get the two metals deposited together it is better to use compounds of similar volatility. One way around this problem is to use a more volatile compound of lithium. One reported synthesis uses a p diketonate of lithium in which lithium is coordinated to 2,2,6,6-tetramethylheptan-3,5-dione (Me3CCOCH2COCMe3) (Figure 3.11). 

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. 

Yttrium tantalate and yttrium niobate-tantalate have good X-ray absorption and are used in X-ray intensifying screens because of their high conversion factor (Section 5.5.4.2.). Substitution of tantalum by small quantities of niobium considerably increases the blue fluorescence when excited by X rays. 

Wohlerite is a niobate of calcium, iron, manganese, sodium, etc., associated with considerable quantities of zirconia and silica. It is found in Norway. Other silicates which contain niobium or tantalum are struverite10 and ilmenorutile.11... 

Zirconium can be removed from the mixed precipitated acids by fusing them with potassium carbonate and extracting the melt with cold water. The niobium and tantalum pass into solution as niobate and tantalate of potassium respectively, while the zirconium remains undissolved as the dioxide, Zr02.4 The method is more suited for the removal of zirconium from niobates than from tantalates.3... 

Addition of excess of potassium thiocyanate to an alkaline solution of a niobate, followed by zinc and concentrated hydrochloric acid, produces a golden-brown colour which may be almost red in the presence of larger quantities of niobium. It is stated that ndther tantalum nor titanium gives any coloration under the same conditions.3 Addition of... 

General.—The compounds of niobium are not so numerous as those of vanadium. The following oxides,1 Nb203, Nb02, Nb205, are known, but only the pentoxide gives rise to salts, viz. the niobates.2 The acid character of niobium pentoxide or niobic acid is very weak the niobates are decomposed, for instance, by carbon dioxide, and are readily hydrolysed to the pentoxide. Niobic acid is, in fact, very comparable in its method of preparation and behaviour to silicic acid and stannic acid. 

Niobium pentafluoride forms colourless, highly refractive prisms density 8 2932 at 18° C. It melts at 75 5° C.f and boils at 217° to 220° C. under a pressure of 760 mm. of mercury. It is extremely hygroscopic and deliquesces rapidly in air. It is reduced by hydrogen at 286° C. in contact with platinum to an unstable lower blue fluoride. Excess of concentrated alkali hydroxide or alkali carbonate solutions attack it with formation of the alkali niobate. It dissolves in toluene, paraffin, carbon bisulphide, and other organic solvents. 

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