Disordered ferroelectrics

Similarly to above glassy systems, the disordered ferroelectrics, polymers and composites are also characterized by slow relaxation processes. Their quantitative measure is complex dielectric permittivity, which can be described by generalized Debye law [29-31] in the form of the following empirical formulas ... 

These materials belong to the group of disordered ferroelectrics where long- and short-range orders coexist. In the spirit of preceding discussion this means that they are in mixed ferroglass phase. As these substances in the mixed phase are very rich in physical properties, they attract much attention from the point of view of their possible nanoelectronics applications. Let us describe briefly some characteristic features of these materials. 

GUnchuk, M.D., Stephanovich, V.A. Random fields and their influence on the phase transitions in disordered ferroelectrics. J. Phys. Condens. Matter 6, 6317-6327 (1994)... 

In second-order or nearly second-order phase transitions, the dielectric dispersion is observed to show a critical slowing-down a phenomenon in which the response of the polarization to a change of the electric field becomes slower as the temperature approaches the Curie point. Critical slowing-down has been observed in the GHz region in several order-disorder ferroelectrics (e.g. Figs. 4.5-8 and 4.5-9) and displacive ferroelectrics (e.g. Fig. 4.5-10). The dielectric constants at the Curie point in the GHz region are very small in order-disorder... 

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). 

Because of very high dielectric constants k > 20, 000), lead-based relaxor ferroelectrics, Pb(B, B2)02, where B is typically a low valence cation and B2 is a high valence cation, have been iavestigated for multilayer capacitor appHcations. Relaxor ferroelectrics are dielectric materials that display frequency dependent dielectric constant versus temperature behavior near the Curie transition. Dielectric properties result from the compositional disorder ia the B and B2 cation distribution and the associated dipolar and ferroelectric polarization mechanisms. Close control of the processiag conditions is requited for property optimization. Capacitor compositions are often based on lead magnesium niobate (PMN), Pb(Mg2 3Nb2 3)02, and lead ziac niobate (PZN), Pb(Zn 3Nb2 3)03. 

Blinc R (2007) Order and Disorder in Perovskites and Relaxor Ferroelectrics. 124 51-67 Boca R (2005) Magnetic Parameters and Magnetic Functions in Mononuclear Complexes Beyond the Spin-Hamiltonian Formalism 117 1-268 Bohrer D, see Schetinger MRC (2003) 104 99-138 Bonnet S, see Baranoff E (2007) 123 41-78... 

Burrows AD (2004) Crystal Engineering Using Multiple Hydrogen Bonds 108 55-96 Bussmann-Holder A, Dalai NS (2007) Order/Disorder Versus or with Displacive Dynamics in Ferroelectric Systems. 124 1-21... 

Dalai NS, Gunaydin-Sen O, Bussmann-Holder A (2007) Experimental Evidence for the Coexistence of Order/Disorder and Displacive Behavior of Hydrogen-Bonded Ferroelectrics and Antiferroelectrics. 124 23-50 Dalai NS, see Bussmann-Holder A (2007) 124 1-21 Daul CA, see Atanasov M (2003) 106 97-125... 

Order/Disorder Versus or with Displadve Dynamics in Ferroelectric Systems... 

Abstract This chapter describes the experimentai compiement of theoretical models of the microscopic mechanism of ferroelectric transitions. We use the hydrogen-bonded compounds as examples, and attempt to show that the new experimental data obtained via recently developed high resolution nuclear magnetic resonance techniques for solids clearly support the hypothesis that the transition mechanism must involve lattice polarizability (i.e. a displacive component), in addition to the order/disorder behaviour of the lattices. 

Keywords Hydrogen-bonded ferroelectrics Order/disorder-displacive effects ... 

Whereas the first microscopic theory of BaTiOs [1,2] was based on order-disorder behavior, later on BaTiOs was considered as a classical example of displacive soft-mode transitions [3,4] which can be described by anharmonic lattice dynamics [5] (Fig. 1). BaTiOs shows three transitions at around 408 K it undergoes a paraelectric to ferroelectric transition from the cubic Pm3m to the tetragonal P4mm structure at 278 K it becomes orthorhombic, C2mm and at 183 K a transition into the rhombohedral low-temperature Rm3 phase occurs. 

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