Ferroelectric properties

The first area of ferroelectric ceramic application was that of capacitor engineering, where the dielectric effect is exploited. Most ceramic capacitors are, in reality, high-dielectric-constant ferroelectric compositions in which the ferroelectric properties (hysteresis loop) are suppressed with suitable chemical dopants while retaining a high dielectric constant over a broad temperature range. Historically, the first composition used for such capacitors was BaTi03 and its modifications, but today lead-containing relaxors and other compositions are also included. 

Ferroelectrics are the prime material for piezoelectric applications in ceramic form, due to the discovery of the electrical poling process that aligns the internal polarization of the crystallites within the ceramic, and causes the ceramic to act in similar fashion to a piezoelectric single crystal. Three categories of applications have 

Ceramics Science and Technology Volume 2 Properties. Edited by Ralf Riedel and I-Wei Chen Copyright 2010 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31156-9 

Transparent ferroelectric single crystals are traditionally used for electro-optic applications. Since the optical transparency was first discovered in lead-lanthanum-zirconate-titanate (PLZT), ferroelectric ceramics have been investigated in great depth such that, today, their characteristics allow them to compete with single crystals for certain electro-optic applications. The electro-optic properties of PLZT compositions are intimately related to their ferroelectric properties. Variations in ferroelectric polarization with an electric field, such as in a hysteresis loop, also affect the optical properties of the material. 

Most of the important ferroelectric properties are closely linked to the variation of spontaneous polarization under the influence of external factors. In this chapter, the important effects are first reviewed related to the key properties of ferroelectrics, such as anisotropy, dielectric hysteresis, and nonlinearity. Some recent trends in the fabrication and application of ferroelectric materials in various forms, structures, and compositions are then highlighted. 

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Potassium Phosphates. The K2O—P20 —H2O system parallels the sodium system in many respects. In addition to the three simple phosphate salts obtained by successive replacement of the protons of phosphoric acid by potassium ions, the system contains a number of crystalline hydrates and double salts (Table 7). Monopotassium phosphate (MKP), known only as the anhydrous salt, is the least soluble of the potassium orthophosphates. Monopotassium phosphate has been studied extensively owing to its piezoelectric and ferroelectric properties (see Ferroelectrics). At ordinary temperatures, KH2PO4 is so far above its Curie point as to give piezoelectric effects in which the emf is proportional to the distorting force. There is virtually no hysteresis. 

Alkaline-Earth Titanates. Some physical properties of representative alkaline-earth titanates ate Hsted in Table 15. The most important apphcations of these titanates are in the manufacture of electronic components (109). The most important member of the class is barium titanate, BaTi03, which owes its significance to its exceptionally high dielectric constant and its piezoelectric and ferroelectric properties. Further, because barium titanate easily forms solid solutions with strontium titanate, lead titanate, zirconium oxide, and tin oxide, the electrical properties can be modified within wide limits. Barium titanate may be made by, eg, cocalcination of barium carbonate and titanium dioxide at ca 1200°C. With the exception of Ba2Ti04, barium orthotitanate, titanates do not contain discrete TiO ions but ate mixed oxides. Ba2Ti04 has the P-K SO stmcture in which distorted tetrahedral TiO ions occur. 

PZN-PT, and YBa2Cug02 g. For the preparation of PZT thin films, the most frequently used precursors have been lead acetate and 2irconium and titanium alkoxides, especially the propoxides. Short-chain alcohols, such as methanol and propanol, have been used most often as solvents, although there have been several successful investigations of the preparation of PZT films from the methoxyethanol solvent system. The use of acetic acid as a solvent and chemical modifier has also been reported. Whereas PZT thin films with exceUent ferroelectric properties have been prepared by sol-gel deposition, there has been relatively Httle effort directed toward understanding solution chemistry effects on thin-film properties. 

Ferroelectric Thin-Film Devices. Since 1989, the study of ferroelectric thin films has been an area of increasing growth. The compositions studied most extensively are in the PZT/PLZT family, although BaTiO, KNbO, and relaxor ferroelectric materials, such as PMN and PZN, have also been investigated. Solution deposition is the most frequentiy utilized fabrication process, because of the lower initial capital investment cost, ease of film fabrication, and the excellent dielectric and ferroelectric properties that result. 

Investigation of the ferroelectric properties of oxyfluoride compounds is also a relatively new but fast-developing area in solid state chemistry [44-46]. 

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. 

FERROELECTRIC PROPERTIES OF NIOBIUM AND TANTALUM FLUORIDE COMPOUNDS... 

The main source of spontaneous polarization in crystals is the relative freedom of cations that fit loosely into the crystal s octahedral cavities. The number of degrees of freedom of the octahedrons affects the spontaneous polarization value and hence influences the crystal s ferroelectric properties. Abrahams and Keve [389] classified ferroelectric materials into three structural categories according to their atomic displacement mechanisms onedimensional, two-dimensional and three-dimensional. 

In other cases, if the fluorination process leads to cardinal changes in the crystal structure of the initial oxide compounds, new compounds with polar structures can be obtained. A demonstrative example of such materials are compounds that belongs to the system Na5(W3 xNbx)09..xF5+x and that have chiolite-type structures, when neither pure fluoride nor oxide display any ferroelectric properties [393 - 395]. 

Tantalum and niobium fluoride compounds that crystallize in coordination-type structures also seem to be perspective candidates for the investigation of ferroelectric properties. Ravez and Mogus-Milancovic [404] showed that some fluoride and oxyfluoride compounds with crystal structures similar to the Re03 type exhibit ferroelastic properties. For instance, ferroelastic properties were found in some solid solutions based on Nb02F and Ta02F [405,406]. 

Ferroelectric properties —ferroelectrics derived from lithium metatantalate... 

The validity of this approach can be demonstrated by the example of several complex fluoride compounds that exhibit ferroelectric properties, such as compounds that belong to the SrAlF5 family [402, 403]. The crystal structure of the compounds is made up of chains of fluoroaluminate octahedrons that are separated by another type of chains - ramified chains. Other examples are the compounds Sr3Fe2Fi2 and PbsWjOgFio. In this case, the chains of iron- or tungsten-containing octahedrons are separated from one another by isolated complexes with an octahedral configuration [423,424]. 

Ferroelectric properties - piezoelectric, pyroelectric and related properties... 

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