Ferroelectric compounds

The ferroelectricity usually disappears above a certain transition temperature (often called a Curie temperature) above which the crystal is said to be paraelectric this is because thermal motion has destroyed the ferroelectric order. Occasionally the crystal melts or decomposes before the paraelectric state is reached. There are thus some analogies to ferromagnetic and paramagnetic compounds though it should be noted that there is no iron in ferroelectric compounds. Some typical examples, together with their transition temperatures and spontaneous permanent electric polarization P, are given in the Table. 

Ferroelectric compounds made by CVD include the following mixed oxides (see Ch. 11, Sec. 11.0) for the CVD reactions). 

The concept of quantum ferroelectricity was first proposed by Schneider and coworkers [1,2] and Opperman and Thomas [3]. Shortly thereafter, quantum paraelectricity was confirmed by researchers in Switzerland [4], The real part of the dielectric susceptibihty of KTO and STO, which are known as incipient ferroelectric compounds, increases when temperature decreases and becomes saturated at low temperature. Both of these materials are known to have ferroelectric soft modes. However, the ferroelectric phase transition is impeded due to the lattice s zero point vibration. These materials are therefore called quantum paraelectrics, or quantum ferroelectrics if quantum paraelectrics are turned into ferroelectrics by an external field or elemental substitution. It is well known that commercially available single crystal contains many defects, which can include a dipolar center in the crystal. These dipolar entities can play a certain role in STO. The polar nanoregion (PNR originally called the polar microregion) may originate from the coupling of the dipolar entities with the lattice [5-7]. When STO is uniaxially pressed, it turns into ferroelectrics [7]. 

Medium-permittivity ceramics are widely used as Class I dielectrics, and in order to be in this category they need to have low dissipation factors. This precludes the use of most ferroelectric compounds in their composition since ferroelectrics have high losses (tan S >0.003), particularly when subjected to high a.c. fields. 

In their analysis of ferroelectric compounds using QSD, Villars etal studied 175 ternary ferroelectric and antiferroelectric oxides. These compounds were divided into three sets F with Tc > 500 K with 50 representatives, F2 with 22 compounds with 300 K < Tc < 500 K, and F3 with 103 representatives containing pseudotemaries and quaternary oxides with F > 500 K. The calculation of AX and AR for... 

In the compounds that are close to PbZr03 in composition, room-temperature structures are orthorhombic (Fig. 12), but the octahedral cations do not uniformly displace parallel to [110]. Rather, for very Zr" cation that displaces parallel to [110], a neighboring Zr" shifts parallel to [TTO]. The net polarization therefore is zero, and the material is classified as antiferroelectric. Antiferroelectric materials also exhibit higher-than-average dielectric constants, but they are not so extreme as those observed in ferroelectric compounds. 

Figure 6.8. Study of the fault stacking density in a ferroelectric compound...

Once the probabilities are known, other physical quantities, which are function of the occupation probabilities, can be calculated from (A) — J2yPy y- or order parameters for order-disorder phase transitions. Different examples will appear in the following. For instance, the orientational contribution to the absolute polarization of the ferroelectric compound pyridinium tetrafluoroborate was estimated from 2H NMR temperature-dependent measurements on the perdeuterated pyridinium cations.116 The pyridinium cation evolves around a pseudo C6 axis, and the occupation probabilities of the different potential wells were deduced from the study of 2H NMR powder spectra at different temperatures. The same orientational probabilities can be used to estimate the thermodynamical properties, which depend on the orientational order of the cation. Using a generalized van t Hoff relationship, the orientational enthalpy changes were calculated and compared with differential scanning calorimetry (DSC) measurements.116... 

The ferroelectric compounds BaTiOa and SrTiOs have both been studied... 

Ferroelectricity depends on temperature. Above 0c ferroelectric behavior is lost and the material becomes paraelectric. The change from the ferroelectric to the non-ferroelectric state is accompanied either by a change in crystal symmetry (e.g., as in BaTiOs) or by an order-disorder transition such as in the organic ferroelectric compound triglycine sulfate (TGS). 

Some properties show hysteresis, and these are not unique functions of the conditions (pressure, temperature) but depend also on the history or the way the state has been arrived at. Examples of such properties are the degree of magnetic or electric polarization in ferromagnetic or ferroelectric compounds at temperatures below the Curie temperature and phase transition such as melting or solidification. The transition temperature depends on whether it is approached from below or above the Curie temperature. The phase transitions in some ferroelectrics are described in Section 4.5. 

Describe the thermal and electromechanical behavior of piezoelectric and ferroelectric compounds. 

Order-disorder transitions are continuous transformations that are characterized by an order parameter that changes continuously from 1 at very low temperatures to 0 at the transition temperature. An ordered solution (in other words a compound) becomes a disordered alloy at temperatures above the transition point. Examples of materials having order-disorder transitions are ionic conductors and ferromagnetic and ferroelectric compounds. Substitutional order transitions involve diffusion and are sluggish those involving rotational disorder are rapid. 

Fig. 4.35 Correlation of temperature dependencies of the molecular tilt 1 and spontaneous polarization P, for a ferroelectric compound DOBAMBC (for the formula see Fig. 3.5a)...

CACT. Center for Advanced Ceramic Technology, at Alfred University, USA. CAD/CAM. Computer Aided Design/ Computer Aided Manufacture Cadmium Carbonate. CdC03. Small amounts (up to 2%) are sometimes added to cadmium selenide (q.v.) red colours to improve their stability. Cadmium Niobate. Cd2Nb207 an anti-ferroelectric compound the Curie temperature lies between - 85°C and -100 °C. 

Lead Niobate. Pb(Nb03)2 a ferroelectric compound having properties that make it useful in high-temperature transducers and in sensing devices. The Curie temperature is 570 = C. 

Lead Tantalate. PbTa205 a ferroelectric compound of interest as an electroceramic. The Curie temperature is 260°C. Lead Titanate. PbTi03 added in small amounts to barium titanate ceramics to improve their piezoelectric behaviour a complex lead titanate-zirconate body (P.Z.T.) finds use as a ceramic component in piezoelectric transducers. The Curie temperature is 490 C. 

Potassium cyanide. KCN used as a neutralizer in vitreous enamelling. Potassium Niobate. KNb03 a ferroelectric compound having a perovskite structure at room temperature. The Curie temperature is 420°C. 

Sodium Tantalate. NaTa03 a ferroelectric compound having the ilmenite structure at room temperature the Curie temperature is approx. 475°C. Of potential interest as a special electroceramic. 

The book is subdivided into three parts. The first three introductory chapters include consideration of the nature of the liquid crystalline state of matter, the physical properties of mesophases related to their electroop-tical behavior, and the surface phenomena determining the quality of liquid crystal cells giving birth to many new effects. The second part (Chapters 5-7) is devoted to various electrooptical effects in nematic, cholesteric, and smectic mesophases including ferroelectric compounds. Here major emphasis is given to explaining the physical nature of the phenomena. The last part (Chapter 8) is a rather technical one. Here recent applications of liquid crystalline materials in electrooptical devices are discussed. 

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