Solid mixed oxides, structure-bonding

Classification of Structure-Bonding in Solid Mixed Oxides... 

Barium titanate is one example of a ferroelectric material. Other oxides with the perovskite structure are also ferroelectric (e.g., lead titanate and lithium niobate). One important set of such compounds, used in many transducer applications, is the mixed oxides PZT (PbZri-Ji/Ds). These, like barium titanate, have small ions in Oe cages which are easily displaced. Other ferroelectric solids include hydrogen-bonded solids, such as KH2PO4 and Rochelle salt (NaKC4H406.4H20), salts with anions which possess dipole moments, such as NaNOz, and copolymers of poly vinylidene fluoride. It has even been proposed that ferroelectric mechanisms are involved in some biological processes such as brain memory and voltagedependent ion channels concerned with impulse conduction in nerve and muscle cells. 

The solid-state Si MAS NMR spectra are informative with respect to structural features of mixed oxides because the peak position of the Si atoms depends on the number of hydroxyls bonded to these atoms (as discussed earlier in Chapter 1) and the structure of the second coordination sphere (Figure 2.94). For instance, an increase in the number of Al atoms in the second coordination sphere of the Si atoms in zeolite ZK-5 (Nakata et al. 1998) results in the down-field shift of the signal (curve 6) i.e., electron shielding of the Si atoms decreases. Additionally, the position of the 04 site (Si(OSi)4) band depends on the composition of the materials (compare curves for ETAS-10 and ZK-5). Triple-quantum Na MAS-NMR spectra of ETS-10 as-prepared and dehydrated showed different heating effects for different sites (Rocha and Anderson 2000). 

It is important to note that the synthesis of many heterocycles is often carried out under acid-catalyzed reactions, so much effort has been put into the search for solid acid catalysts (Rosati et al., 2007 Dhakshinamoorthy et al., 2011 Sreekumar and Padmakumar, 1998 Kandarpa et al., 2011 Krishnakumar and Swaminathan, 2011 Huang et al., 2008). From this point of view, catalysis by heteropoly acids (HPAs) and related compounds is a field of increasing importance worldwide. To avoid the use of conventional acid catalysts (sulfuric, phosphoric, and hydrofluoric acids and boron trifluoride) and the related environmental pollution and corrosion problems (Vdzquez et al., 2002), insoluble solid acid catalysts such as HPAs can be used. HPAs are mixed oxides composed of a central ion or heteroatom, generally P, As, Si or Ge, bonded to an appropriate number of oxygen atoms and surrounded by a shell of octahedral MOg units. HPAs with Keggin structure and related polyoxometalates are quite common and are represented by the formula Hg [XM,204o], where X is the... 

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