Disorder compositional

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. 

Fig. 57. Cocrystallized [TpAnt]CoNCS and [TpAnt]Tl. Reprinted from Polyhedron, 14, Han, R., Parkin, G., and Trofimenko, S The tris[3-(9-anthryl)pyrazol-l-yl]hydroborato ligand, [TpAnt] Compositional disorder between a vacancy and a chain of three atoms, p. 387, copyright 1995, with kind permission from Elsevier Science Ltd, The Boulevard, Langford Lane, Kidlington 0X5 1GB, UK.

In this section, we will examine some features of these two phenomena—room-temperature reversible photodarkening and photocrystallization—in amorphous semiconductors films of Sbj Sei j . For the starting material, pure amorphous Se was chosen. One of the elemental amorphous materials, Se may be extremely suitable for discussing essential features and relationship (if such exists) between the two phenomena chosen for discussion. In addition, the effect of small amounts of antimony (a few percent) on photodarkening and photocrystallization of a-Se is especially interesting— not only from the point of view of compositional disordering, but also because of desirable recording properties and peculiarities of electronic transport for amorphous Sb Sci films [25]. 

It should be stressed that the diffraction methods do not provide complete characterization of lattice distortions and ionic shifts in relaxors due to the compositional disorder of these materials and nanometric scale of polar order. Thus, local methods such as magnetic resonance and, in particular, NMR can be extremely useful in this case. In NMR experiments, the nuclei are sensitive to their local environment at a distance less than 1-2 nm. In addition, NMR operates at a much longer time scale (105-108 s) in comparison with the neutron or X-ray... 

Uncontrolled compositional disorder comes either from chemical impurities (e.g., oxidation of the bath during crystal growth, partial degradation of one of the reactants during the synthesis, etc.) or from fluctuations in reactant proportions (nonuniform stoichiometry). As stated above, disorder may not affect the accuracy of structure studies however, some synthesis or crystal growth process may occasionally lead to unusually large amounts of chemical impurities, which can both modify the physical properties of the material significantly and/or lead to specific diffraction features. 

The nonstoichiometric compounds just described are ionic materials with compositional disorder. A related type of disorder is exhibited by an alloy, a mixture of elements that displays metallic properties. 

Fig. 39a. Chemical architecture of a diblock copolymer. A diblock copolymer consists of a polymerized sequence of A monomers (A-Block) covalently attached to a similar sequence of B-monomers. b The microphase separation transition occurs when a compositionally disordered melt of copolymers (right side) transforms to a spatially periodic compositionally inhomogeneous phase (left side) on lowering the temperature. For nearly symmetric copolymers (composition f near fas 1/2) the ordered phase has the lamellar structure shown. From Fredrickson and Binder [61]...

Figure Al.3.28. Examples of disorder (a) perfect crystal, (b) compositional disorder, (c) positional disorder which retains the short-range order and (d) no long-range or short-range order.

In the semiconducting alloys, on the other hand, ideality of structure, i.e., local satisfaction of valence requirements, implies that the connectivity of the network varies locally as the valence of the constituents vary. Thus, in addition to the translational disorder of the elements and compounds, there is compositional disorder at the nodes of the network and additional translational disorder associated with the randomness of connectivity. This increased disorder could broaden the tails until they overlap, (Cohen (1969)) as shown in Figure 3.5. Such overlapping may also be the case for some liquid semiconductors because of their reduced short-range order. The... 

Beaglehole and Zavetova (1970) found that the fundamental absorption band of a-Si is similar to that of a-Ge the maximum is at 3.1 eV and the band is narrower (at 2 = 10 its width is 2.1 eV as compared to 3 eV in a-Ge). They also observed a smooth shift of the band in Ge-Si alloys with the change in composition from Ge to Si without any additional broadening due to the compositional disorder. 

The observed dielectric properties of the relaxor ferroelectrics are due to the compositional disorder of the B-site Bi, B ) cation distribution and the associated... 

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