Giant unit cell

An important contribution to the structure analysis of intermetallic phases in terms of the coordination polyhedra has been carried out by Frank and Kasper (1958). They described several structure types (Frank-Kasper structures) as the result of the interpenetration of a group of polyhedra, which give rise to a distorted tetrahedral close-packing of the atoms. Samson (1967, 1969) developed the analysis of the structural principles of intermetallic phases having giant unit cells (Samson phases). These structures have been described as arrangements of fused polyhedra rather than the full interpenetrating polyhedra. 

In a cubic giant unit cell 24 tetrameric (NaMe)4 units form a zeolitic host lattice. This structure is related to that of methyllithium, but the arrangement of the tet-ramers is more complicated. It results in the presence of large cavities, in which (LiMe)4 units can be intercalated up to the structurally predetermined molar ratio of 3 1. In this sense the host-guest compound (NaMe)4 [(LiMe)4]4 x < 0.333), 39, was termed the first organometallic supramolecular compound . 

The lattice parameters are large, exceeding, say, 1.0 nm. Correspondingly, the unit cells are of large volume (frequently referred to as giant unit cells ) and contain many tens up to several thousands of atoms. 

The study of QC surfaces has led to interest in the surfaces of related CMAs. QCs typically exist in a narrow composition region of the phase diagram due to the Hume-Rothery constraint of specific valence electron to atom ratio, which is related to electronic stabilization of QCs [196-198]. In the neighborhood of this composition region, phases with giant unit cells and local atomic order related to that of the QC can usually be found. The surfaces of these approximant phases offer the possibility of exploring surface structure and properties as a function of increasing complexity, which can be most simply defined in terms of atoms f>er unit cell. 

Statistics indicate that in late 1978, the American Red Cross reported a demand for thawed, deglycerolized human red cells of nearly 100,000 units per year, By May 1983, the demand was only approximately 42,000 units. Thus, it now appears that the need for frozen red cells was mainly found in filling rare donor blood types. Chaplin summarizes—future developments may yetprove previously frozen red cells to be the sleeping giant ofred-cell replacement therapy for the present, we must be content with the minor but crucial role in which they have proved their worth beyond doubt. 

Electrofusion has been successful in all types of cells tested to date, including microbe and plant protoplasts, mammalian cells, and sea urchin ova. One can (a) fuse unlike cells to create hybrid cells (b) fuse like cells to form larger entities such as giant cells 100 to 1000 times the volume of individual unit cells and (c) help drive external objects or chemical agents such as DNA into cells. 

In a sodium chloride crystal, the Na" and Cl ions are arranged in a giant lattice structure. The building brick of this structure is a unit cell is as shown in Fig 4.2. The larger spheres represent Cl ions, whereas the smaller spheres represent Na" ions. A crystal of sodium chloride consists of many billions of these unit cells stacked together in the lattice. 

There are four lattice points in the face-centred unit cell, and the motif is two atoms, one atom at (0,0,0) and one at (, , ). The structure is adopted by diamond and, in it, each carbon atom is bonded to four other carbon atoms that are arranged tetra-hedrally around it (Figure 5.20). The bonds, of length 0.154nm, are extremely strong sp hybrids. The crystal can be regarded as a giant molecule. 

The application of the method to trimeric units with the same ligands as above (BDC and BTC) in a 1 1 ratio for BDC and 2 3 for BTC leads to a whole series of plansible and very open MOFs with hitherto unknown topologies for most of them. Among all the discovered topologies, only three present reasonable lattice energies, bnt only one fits with the experimental data of powder diffraction. This topology is of particular interest for the unprecedented giant cubic cells they provide (380,000 for BTC and 706,000 for BDC), far beyond all the known ones. 

The cubic unit cell of YBgg-type compounds with 1632 boron atoms consists of 13 giant (B 12)13 icosahedra and 8 nonicosahedral B42 units [stmctural formula Bi2(Bi2)i2(B42)8] with the metal atoms statistically distributed on defined interstitial sites (Fig. 6) (46,47). The B42 units are clusters consisting of 80 boron sites with occupancies ranging between 28 and 71% (48). The homogeneity range is assumed to be considerable (YB , 20 < n < 100). Besides Y, most of the lanthanide and some actinide atoms are known to form this sUiicture (see Refs. 3,49, and 50). 

Pure metallic elements, for example, aluminum, copper, or iron, usually have atoms that fit in a few symmetric patterns. The smallest repetitive unit of this atomic pattern is the unit cell. A single crystal is an aggregate of these unit cells that have the same orientation and no grain boundary, ft is essentially a single giant grain with an orderly array of atoms. This unique... 

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