Superstructures substitutional

Equal positions for all of the atoms Ordered substitution (superstructure formation) SmjGej —> CejSCjSi CeNij— CejCOgSi, etc. 

As a conclusion to this section, notice that a systematic description of ordering processes in alloys and of the superstructures which can be generated has been presented, for instance, by Khachaturyan (1983) in the framework of a theoretical treatment of structural transformation in solids. Two groups of superstructures have been specially considered substitutional and interstitial. 

A first group of superstructures, described in several paragraphs of this chapter and of Chapter 7, must be mentioned these include the types tP2-AuCu(I), cP4-AuCu3 and tP4-Ti3Cu which can be considered face-centred cubic-based substitutional ordered superstructures. 

Cu-derivative, substitutional and interstitial superstructures. As discussed in 3.8.1 ff, the Cu-type structure is also an important reference structure because it may be considered the ancestor of several derivative structures. 

The W body-centred cubic structure can be compared with the simple cubic CsCl-type structure (which can be obtained from the W type by an ordered substitution of the atoms) and with the MnCu2Al-type structure ( ordered superstructure of the CsCl type) see Fig. 3.31 and notice the typical eight (cubic) coordination. 

The often mentioned relations between the structure types of cryolite and perovskite (page 41) may be explained best with the example of the elpasoHte type. The elpasoUte structure is really a superstructure of the perovskite-lattice, generated by substituting two divalent Me-ions in KMeFs by two others of valency 1 (Na) and 3 (Me) resp. The resulting compound K (Nao.5Meo.5)F3 crystallizes with an ordered distribution of Na+ and Me + because of the differences in size and charge of the ions. Thus to describe the unit cell the lattice constant of the perovskite ( 4 A) has to be doubled to yield that of the elpasohte structure ( 8 A). 

The crystal chemistry of BajRC C has been systematically studied by single-crystal and powder diffraction methods with R = La, Pr,... Yb, in addition to the conventional yttrium compound [(52)(53) (54) and references therein]. With the exception of La, Pr, and Tb, the substitution of Y with rare-earth metals has little or no effect on the superconductivity, with the values of Tc ranging from 87 to 95K. Also, a relatively small change is observed in the cell constants of these compounds. The La, Pr, and Tb-substituted materials are not superconductors. A detailed structural analysis of the Pr case (52) did not show any evidence of a superstructure or the presence of other differences with the atomic configuration of the yttrium prototype. 

The ternary iron oxides, as exemplified by the iron-niobium system, offer an opportunity to obtain single-phase, conducting n-type iron oxides in which the conductivity can be controlled by means of chemical substitution. At first glance, FeNbO and FeNb Og might appear to be very different materials. Yet as MM O and MM Og they merely represent superstructures of the basic a-PbO. structure obtained under the conditions of preparation (7 ). Consequently, they form a solid solution in which the two valence states of iron are uniformly distributed throughout a single homogeneous phase (j3). 

In general, traditional electrode materials are substituted by electrode superstructures designed to facilitate a specific task. Thus, various modifiers have been attached to the electrode that lower the overall activation energy of the electron transfer for specific species, increase or decrease the mass transport, or selectively accumulate the analyte. These approaches are the key issues in the design of chemical selectivity of amperometric sensors. The long-term chemical and functional stability of the electrode, although important for chemical sensors as well, is typically focused on the use of modified electrodes in energy conversion devices. Examples of electroactive modifiers are shown in Table 7.2. 

However, the particular synthetic requirements in the preparation of conjugated polymers have thus far severely limited the number of similarly hierarchically structured examples. Pu et al. reported different types of conjugated polymers with fixed main-chain chirality containing binaphthyl units in their backbone which exhibited, for example, nonlinear optical activity or were used as enantioselective fluorescent sensors [42—46]. Some chirally substituted poly(thiophene)s were observed to form helical superstructures in solution [47-51], Okamoto and coworkers reported excess helicity in nonchiral, functional poly(phenyl acetylenejs upon supramolecular interactions with chiral additives, and they were able to induce a switch between unordered forms as well as helical forms with opposite helical senses [37, 52, 53]. 

Frechet et alJ62a] further investigated the functionalization of the poly(benzyl ether) dendritic superstructure or framework (Scheme 5.19). For example, treatment of monoalcohol 70 [G-4]-OH fourth tier alcohol) with a 1 3 mixture of n-butyllithium and potassium f< r/-pentoxide (THF, hexane, - 85 °C) gave the polypotassium salt 75. Quenching with electrophiles that included D20, Me3SiCl, Ci8H37Br or C02 provided the functionalized dendrimer 76. However, electrophilic addition was not site-specific, although multisite substituted dendrimers were produced. 

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