Niobates

Koppite, natural pyrochlorc from Schclingen, Kaiserstuhl. 

Properties 99.9% pure (manufacturer), BET specific surface area 1.2 m-/g, specific density 7967 kg/m [147]. 

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Nmol AA62 Extra NmolAD31 Nmol 2021 Extra Niobate glass-ceramics Niobium... 

Typical electrostrictive materials include such compounds as lead manganese niobate lead titanate (PMN PT) and lead lanthanium 2irconate titanate (PLZT). Electrostriction is a fourth-rank tensor property observed in both centric and acentric insulators (14,15). 

Simple ABO compounds in addition to BaTiO are cadmium titanate [12014-14-17, CdTiO lead titanate [12060-00-3] PbTiO potassium niobate [12030-85-2] KNbO sodium niobate [12034-09-2], NaNbO silver niobate [12309-96-5], AgNbO potassium iodate [7758-05-6], KIO bismuth ferrate [12010-42-3], BiFeO sodium tantalate, NaTaO and lead zirconate [12060-01 -4], PbZrO. The perovskite stmcture is also tolerant of a very wide range of multiple cation substitution on both A and B sites. Thus many more complex compounds have been found (16,17), eg, (K 2 i/2) 3 ... 

There is often a wide range of crystalline soHd solubiUty between end-member compositions. Additionally the ferroelectric and antiferroelectric Curie temperatures and consequent properties appear to mutate continuously with fractional cation substitution. Thus the perovskite system has a variety of extremely usehil properties. Other oxygen octahedra stmcture ferroelectrics such as lithium niobate [12031 -63-9] LiNbO, lithium tantalate [12031 -66-2] LiTaO, the tungsten bron2e stmctures, bismuth oxide layer stmctures, pyrochlore stmctures, and order—disorder-type ferroelectrics are well discussed elsewhere (4,12,22,23). 

Certain glass-ceramic materials also exhibit potentially useful electro-optic effects. These include glasses with microcrystaUites of Cd-sulfoselenides, which show a strong nonlinear response to an electric field (9), as well as glass-ceramics based on ferroelectric perovskite crystals such as niobates, titanates, or zkconates (10—12). Such crystals permit electric control of scattering and other optical properties. 

Perovskites have the chemical formula ABO, where A is an 8- to 12-coordinated cation such as an alkaU or alkaline earth, and B is a small, octahedraHy coordinated high valence metal such as Ti, Zr, Nb, or Ta. Glass-ceramics based on perovskite crystals ate characteri2ed by their unusual dielectric and electrooptic properties. Examples include highly crystalline niobate glass-ceramics which exhibit nonlinear optical properties (12), as well as titanate and niobate glass-ceramics with very high dielectric constants (11,14). 

Only certain types of crystalline materials can exhibit second harmonic generation (61). Because of symmetry considerations, the coefficient must be identically equal to zero in any material having a center of symmetry. Thus the only candidates for second harmonic generation are materials that lack a center of symmetry. Some common materials which are used in nonlinear optics include barium sodium niobate [12323-03-4] Ba2NaNb O lithium niobate [12031 -63-9] LiNbO potassium titanyl phosphate [12690-20-9], KTiOPO beta-barium borate [13701 -59-2], p-BaB204 and lithium triborate... 

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. 

Fusion with caustic soda at 500—800°C in an iron cmcible is an effective method for opening pyrochlores and columbites (20). The reaction mixture is flaked and leached with water to yield an insoluble niobate which can be converted to niobic acid in yields >90 wt% by washing with hydrochloric acid. 

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. 

Nloha.tes, Niobic acid is amphoteric and can act as an acid radical in several series of compounds, which are referred to as niobates. Niobic acid is soluble in solutions of the hydroxides of alkaH metals to form niobates. Fusion of the anhydrous pentoxide with alkaH metal hydroxides or carbonates also yields niobates. Most niobates are insoluble in water with the exception of those alkaH metal niobates having a base-to-acid ratio greater than one. The most weU-known water-soluble niobates are the 4 3 ad the 7 6 salts (base acid), having empirical formulas MgNb O c, (aq) and M24Nb2202y (aq), respectively. The hexaniobate is hydrolyzed in aqueous solution according to the pH-dependent reversible equiHbria (130), when the pH is ca 9. 

Sodium metaniob ate (1 1) [67211-31-8] Na20 Nb20 7H20 or Na2Nb20g-7H20, separates as colorless triclinic crystals as a result of concentrating the mother Hquor from the preparation of the 7 6 sodium niobate by spontaneous evaporation. It also can be obtained by fusion of the anhydrous pentoxide in sodium hydroxide or carbonate. 

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... 

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