Ferroelectrics order-disorder type

Triglycine sulfate (TGS) - (NH2CH2COOH)3.H2S04. It belongs to the ferroelectric species 2/ot 2 with relatively low Cmie temperature 49° C. It is model material for ferroelectric order-disorder type of phase transition. 

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). 

Keywords Anharmonic effects Displacive phase transition Isotope effects KDP-type ferroelectrics Order-disorder phase transition... 

Initially, it appeared that the phase transitions are purely of the order-disorder type [24]. However, it soon became apparent that atomic displacements also contribute to these phase transitions. These displacements are easy to identify, when one compares the molecular arrangements in the ordered -OH 0= bonds in Fig. 1, with the arrangement of these molecules linked by the disordered bond in Fig. 2. The hydrogen-bonded molecules/ions must rotate before the two H-sites become symmetry-equivalent in the paraelectric phase above Tc- These rotations, usually of few degrees, can be termed as angular displacements. In other words, these angular displacements measure the distortions of the ferroelectric structure where the H-atom ordered from the paraelectric structure where the H-atom is dynamically disordered in two equivalent sites. 

Gradient coefficients > 0 and q > 0 the expansion coefficient an>0 for the second order phase transitions. Coefficient ai(T) = ar T — Tc), E is the transition temperature of a bulk material. Note, that the coefficient flu for displacement type ferroelectrics does not depend on T, while it is temperature dependent for order-disorder type ferroelectrics (see corresponding reference in [117]). Eq is the homogeneous external field, the term Ed (P3) represents depolarization field, that increases due to the polarization inhomogeneity in confined system. Linear operator Ed P3) essentially depends on the system shape and boundary conditions. Below we consider the case when depolarization field is completely screened by the ambient free charges outside the particle, while it is nonzero inside the particle due to inhomogeneous polarization distribution (i.e., nonzero divP 0) (see Fig. 4.35b). 

Ferroelectricity is caused by a cooperative interaction of molecules or ions in condensed matter. The transition to ferroelectricity is characterized by a phase transition. Depending on the mechanism of how the molecules or ions interact in the material, we can classify the ferroelectric phase transitions and also the ferroelectric materials themselves into three categories (I) order-disorder type, (II) displacive type, and (III) indirect type. In the order-disorder type (I), the spontaneous... 

Resonant behavior of the dielectric dispersion in ferroelectric crystals of the displacement-type appears usually in the far infrared frequency range. For the ferroelectrics of the order-disorder type it appears in the microwave frequency range. 

Ferroelectrics are electrically sensitive polar crystals. Oi]gaiuc polymers, in either glass or crystal, pack by very cohesive forces and are easily deformed by an exiertui field. Therefore, a polymer with large dipoles such as a polar pdymer can be a ferroelectric of the order-disorder type if its structure is contr ed appropriately. 

The phonon contribution to the heat capacity is the most important one, but others also occur. As noted in Section 2.3.7, the heat capacity due to free electrons in metals is small but significant heat capacity changes accompany phase changes, such as order-disorder changes of the type noted with respect to ferroelectrics (Sections 11.3.5 and 11.3.6), or when a ferromagnetic solid becomes paramagnetic (Sections 12.1.2 and 12.3.1). 

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