Perovskite Niobates and Tantalates

Preparation of Ferroelectric Ceramics 17.3.11.3. Other Ferroeiectctric Ceramics 17.3.11.3.2. Perovskite Niobates and Tantalates. 

Most niobates and tantalates, however, are insoluble and may be regarded as mixed oxides in which the Nb or Ta is octahedrally coordinated and with no discrete anion present. Thus KMO3, known inaccurately (since they have no discrete MO3 anions) as metaniobates and metatantalates, have the perovskite (p. 963) stmcture. Several of these perovskites have been characterized and some have ferroelectric and piezoelectric properties (p. 57). Because of these properties, LiNb03 and LiTa03 have been found to be attractive alternatives to quartz as frequency filters in communications devices. 

With the exception of a few insoluble lanthanide niobates and tantalates, e.g., ScNb04,5 which contain discrete, tetrahedral MO ions, the coordination number of Nbv and Tav with oxygen is essentially always 6. The various niobates and tantalates are really mixed metal oxides. Thus, for example, the M XO, compounds are perovskites (page 55). 

In late 1990s it was shown that the proton transport can be also realized in perovskite-related compounds with structural oxygen vacancies. The niobates and tantalates of alkaline-... 

Animitsa 1., Denisova T., Neiman A., Nepryahin A., Kochetova N., Zhuravlev N., Colomban Ph. States of H -Containing Species and Proton Migration Forms in Hydrated Niobates and Tantalates of Alkaline-Earth Metals with a Perovskite-Related Structure// Solid State Ionics. 2003. V.62-163,73-81. 

Perovskites and related compounds Barium titanate Lead titanate Potassium niobate Potassium tantalate niobate Lithium niobate Lithium tantalate Barium titanium niobate Ba-Na niobate ( Bananas )... 

All alkoxide routes appear to be limited to obtaining PNM [88a] and lithium or potassium niobate or tantalate [102a]. For the latter, the formulation of the mixed-metal alkoxide MM (OR)6, M = Nb, Ta, is in agreement with that of the material. Epitaxial thin films of LiNbOa on sapphire could be obtained [103]. The use of a mixed-metal MgNb2(OR)i2 species (R = Et [105], C2H40Me [104]) has been shown to promote the formation of the perovskite phase for PNM. 

Potassium tantalate-niobate [K(Ta Nbi jc)03, KTN] is one of the ferroelectric materials with the perovskite structure, and is a sohd solution of potassium tantalate (KTaOs) and potassium niobate (KNbOs). The Ciuie temperature of KTN for the cubic to tetragonal transition varies with Ta/Nb ratio, and is lowered with increasing Ta substitution (Triebwasser, 1959). The ferroelectric properties of KTN, therefore, can be controlled by the Ta/Nb ratio. The nonferroelectric cubic phase of KTN atx= 0.65 is known to show photorefractive effect based upon a large quadratic electro-optic coefficient at room temperature (Gausic, 1964 Orlowski, 1980). 

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). 

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