Solid superstructure

Fig. 2.49. Examples of experimental (solid lines) and calculated (dashed lines) LEED l-V curves for the p(v x v ) R19° superstructure of benzene on Ru(OOOl). The individual Rp factors are indicated in the figure [2.263],...

Although the corrosivity may not be high provided the condensed moisture remains uncontaminated, this rarely happens in practice, and in marine environments sea salts are naturally present not only from direct spray but also as wind-borne particles. Moreover, many marine environments are also contaminated by industrial pollution owing to the proximity of factories, port installations, refineries, power stations and densely populated areas, and in the case of ships or offshore installation superstructures by the discharge from funnels, exhausts or flares. In these circumstances any moisture will also contain S, C and N compounds. In addition, solid pollutants such as soot and dust are likely to be deposited and these can cause increased attack either directly because of their corrosive nature, or by forming a layer on the surface of the metal which can absorb and retain moisture. The hygroscopic nature of the various dissolved salts and solid pollutants can also prolong the time that the surface remains moist. 

The diaminopyridodipyrimidinedione 73 has been synthesised as a cytosine-like Tecton designed for self assembly into a helical superstructure. Self recognition occurs in the solid state <96MI02 96CA(125)221771 >. 

Spectroscopy has also been a key to understanding the universe. Astronomers attach spectrometers to their telescopes to study the makeup of the sun and the stars and to measure their speeds relative to Earth and one another. In fact, spectroscopy is one of sciences most valuable tools. It owes a huge debt to the many experimenters who recorded the spectral lines of hundreds of elements and compounds. But their work is the superstructure that rests on the solid foundations laid by Bohr, Schrodinger, and their colleagues. 

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. 

Like the Au-Cu system, also the alloy system Cu-Pt is characterized by a phase diagram with the face-centred cubic continuous solid solution stable at high temperature and, for different composition ranges, a number of ordered superstructure phases stable at lower temperatures. CuPt(I), for instance, is a complex, slightly... 

The high temperature superconductivity discovered in the Bi-Sr-Ca-Cu-O system was found to be related to a homologous series of compounds with an idealized formulation Bi2Sr2Ca iCu 02n+4 (n = 1 to 3 or more). The n = I, 2, 3 phases have Tc values of 10-20, 90 and 11 K, respectively. The presence of superstructure modulations which are, in general, incommensurate with the basic structure was first discovered by HRTEM and ED (Shaw et al 1988, Gai et al 1988). The periodicity of the modulation is found to be about 4.7 multiplied by the a-lattice parameter. The compounds can be prepared from solid state reactions of the component oxides in stoichiometric proportions and heating between 800-900 °C in air. 

Perovskites constitute an important class of inorganic solids and it would be instructive to survey the variety of defect structures exhibited by oxides of this family. Nonstoichiometry in perovskite oxides can arise from cation deficiency (in A or B site), oxygen deficiency or oxygen excess. Some intergrowth structures formed by oxides of perovskite and related structures were mentioned in the previous section and in this section we shall be mainly concerned with defect ordering and superstructures exhibited by these oxides. 

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). 

ZrGa2 is orthorhombic with four molecules per unit cell, a = 12.894, b = 3.994, and c = 4.123 A. It is a slightly distorted superstructure of close-packed layers. In the a direction there are three fee pseudocells. A close-packed layer normal to 111 of the pseudocell is shown in Figure 9.16. The sequence of close-packed layers is ABC, but actually ABC A B C A"B"C" because of changing environments causing by partial filling by Zr and Ga. Positions B, C, B, C, A", B", and C" are shown by solid dots. For the close-packed layer shown, one horizontal row is fully occupied by Ga and one-half occupied by Ga in the next two rows. In rows parallel to the other cell faces one row is filled... 

Mn2Au5 is monoclinic with two molecules per unit cell, a = 9.188, c = 6.479 A, and (3 = 97.56°. It is a superstructure of face-centered subcells. In Figure 9.17a we can see the face-centered subunit (dotted lines) and the smaller body-centered subunits (solid lines). There are close-packed layers parallel to 111 planes of the pseudocell. The close-packed layers are in an ABC sequence with 21 layers repeating (Figure 9.17b). The notation is 3 7P(m)(2 5). 

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