Structure superstructures

STRUCTURE FAMILIES DEGENERATE STRUCTURES, DERIVATIVE STRUCTURES, SUPERSTRUCTURES ... 

An example where, due to ordering, we observe perhaps in a more immediate way, the increase of the unit cell size (formation of a multiple cell) is the MnCu2Al-type structure (representative of the so-called Heusler alloys) which can be considered a derivative structure (superstructure) of the cP2-CsCl type, which in turn is a superstructure of the W-type structure, corresponding to a non-primitive cubic cI2... 

In the preceding paragraphs examples of a number of so-called superstructures have been considered. Generally, it has been observed that a derivative structure has fewer symmetry operations than the reference structure it has either a larger cell or a lower symmetry (or both) than the reference structure. Typically the passage from the reference structure to the derivative structure (superstructure) may be related to the fact that a set of equipoints of a certain structure (the reference one) has to be subdivided into two (or more) subsets in order to obtain the description of the other structure. The structure of the Cu type (cF4 type), for instance, corresponds to 4 Cu atoms in the unit cell, placed in 0, 0, 0 14, 14, 0 14, 0, 14 0, 14, 14, whereas in the cP4-AuCu3 type structure the same atomic sites are subdivided, in another space group, into two sets with an ordered distribution of the two atomic species (1 Au atom in 0, 0, 0 and 3 Cu atoms in 14, 14, 0 14, 0,14 0,14,14). 

In addition to the binary nitrides described here, some ternary nitrides have been prepared, for example, Li5TiN3, Li7NbN4, and Li9CrNs, all with the fluorite type of structure (superstructures in most cases), and alkaline-earth compounds with Re, Os, Mo, or W(e.g. Sr9Re3Nio, CasMoNs). ... 

At variance with the data of Rieger and Parthe (1969a) obtained from arc-melted samples, landelh (1983) observed the formation of GdCuSi with the ordered Ni2In-type of structure [superstructure of AlBj-type, Pbj/mmc, a = 4.168(1), c = 7.573(4)]. The superstructure reflections were said to be very faint and were obtained from arc-melted samples after annealing at 750°C for 8—12 d. The Nijin phase was thus concluded to be a low-temperature modification. For details in sample preparation, see La—Cu-Si. 

Structure-superstructure relationships between structure types of binary and ternary intermetallic indium compounds... 

YbCde one cadmium atom occupies the 16/ site by about one half. These inconsistencies might be due to a variable stoichiometiy or different qualities of the structure solution. On the other hand, in the temaiy compound YbAg2ln4, the silver atoms occupy the site 48/ by about one sixth. It should be mentioned, that the mixed site occupancies actually do not result in a change of the general stmctural motif. This manifests the view of the structure-superstructure relationship at least in the sense of Kripyakevich (1977). The YbAg2ln4 structure has one more peculiarity the sites 16/ and 24g are occupied by the atoms Agl, Ag2, Ag3, and Ag4, respectively, on spht positions, but with a total occupancy of 100%. 

WAXS patterns at 0.6, 0.9 and 2.9 mol% H2O are shown in Fig. 12. If we assume a smaller unit cell and two different kinds of modulated structures (superstructures), the observed 26 values are almost the same as the calculated ones. The smaller orthorhombic unit cell is expressed by Oo, hd and cd Blue circles repaesent the superceU of ad hd 2co (Z=8), while red circles indicate another superceU of 2ad 6d 2co (Z=16) (Fig. 12). Unit cells and the volume per four [DEMEJpFJ imits, V4, of [DEME][BF4]-H20 are listed in Table 1. A significant finding is that V4 at 0.9 mol % is distinctly smaU. It should be noticed that the twin-related domain at 0.9 mol% H2O (//o 7) is formed with two accompanying kinds of superstructures (Z=4m m=2 and 4) and a volume contraction (smaU V4). 

Figure 16.26 shows a pair of composite curves divided into enthalpy intervals with a possible superstructure shown for one of the intervals. The structure is created by splitting each hot stream... 

While a superstructure based on the structure in Fig. 16.26 allows for many structural options, it is not comprehensive. Wood, Wilcox, and Grossmanr showed how direct contact heat transfer by mixing at unequal temperatures can be used to decrease the number of units in a heat exchanger network. Floudas, Ciric, and Grossman showed how such features can be included in a heat exchanger network superstructure. Figure 16.27 shows the structure from Fig. 16.26 with possibilities for direct contact heat transfer included. In the... 

Alternative superstructures to those in Figs. 16.26 and 16.27 can be developed. On the one hand, it is desirable to include many structural options to ensure that all features which are candidates for an optimal solution have been included. On the other hand, including more and more structural features increases the computational load dramatically. Thus care should be taken not to include unnecessary features in the superstructure. 

Asymmetric simhaiity measures allow fuzzy super- and substructure searching. A substructure search is defined as looking for structures containing the given query and a superstructure search is defined as looking for structures embedded in the given query. In both cases asymmetric local similarity is estimated. 

Quite often, NBR adhesives are used to bond various kinds of gasketing (cork, fibre, foam, rubber, metal) to rigid superstructures, such as aircraft. Films cast from solution are often used to fabricate honeycomb structures for aircraft. 

In the last decade two-dimensional (2D) layers at surfaces have become an interesting field of research [13-27]. Many experimental studies of molecular adsorption have been done on metals [28-40], graphite [41-46], and other substrates [47-58]. The adsorbate particles experience intermolecular forces as well as forces due to the surface. The structure of the adsorbate is determined by the interplay of these forces as well as by the coverage (density of the adsorbate) and the temperature and pressure of the system. In consequence a variety of superstructures on the surfaces have been found experimentally [47-58], a typical example being the a/3 x a/3- structure of adsorbates on a graphite structure (see Fig. 1). 

AufbaU) m. building up, synthesis structure mounting building, construction, erection superstructure display. auftiauebeU) v.t., i. r. swell up, puff up. aufbaueU) v.t. Wld up, sjmthesize erect. — aufbauend) p.a. sjmthetic constructive. auftiaumeU) v.i. show (metallic) luster. — v.r. bear up, struggle. 

If the electrode potential is further reduced to h-350 mV, a hexagonal superstructure with a periodicity of 2.4 0.2 nm is observed. With respect to the interatomic distances in the Au(lll) structure at the surface, this corresponds - within the error limits - to an 8 X 8 superstructure (Figure 6.2-9). 

PET fibers and filaments are characterized by a fibrillar superstructure that corresponds to the general concept of the fibrillar structure of synthetic fibers. The fibrillar... 

LEED, namely one with a, c(2x2) and one with a, p(2x2) superstructure. They are compatible with CusPt and CusPta layers. The first atomic layer was in both cases found by means of photoemission of adsorbed xenon to be pure copper. Details of the experimental work can be found in ref. 9 and 10. A schematic view of both structures can be seen in figure 1. Both consist of alternating layers of pure copper and of mixed composition. In the CuaPt case, the second and all other evenly numbered layers have equal numbers of copper and platinum atoms, whereas in the CusPta case the evenly numbered layers consist of thrice as many platinum as copper atoms. 

---