W type structure

Cr3Si-type structure W + / AgMgAs-type structure F + F + F" CaTi03-type structure P + P + J Cu2Mg-type structure T + D. For a few element structures Cu type structure F W-type structure 7 a-Po-type structure P Mg-type structure E C-diamond-type structure D. 

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... 

Notice, for instance, the similar shape of the f3 phase field in all three systems the [3 phase is the cI2-W type structure. 

Their normal crystal structure, at ambient conditions, corresponds to the body-centred cubic cI2-W-type structure. At very low temperatures, the close-packed hexagonal hP2-Mg-type structure has been observed for Li and Na, while for Rb and Cs the face-centred cubic close-packed cF4-Cu-type structure is known at high pressure. No polymorphic transformation has been reported for potassium. 

Titanium, zirconium and hafnium in normal conditions crystallize in the hexagonal close-packed structure (a modification) with a c/a slightly smaller than the ideal one c/a = 1.587 (Ti), 1.593 (Zr) and 1.581 (Hf). At high temperature they have the bcc W-type structure ((3 modification). High-pressure transformations are known (Tables 5.21-5.23). 

The 5th group metals a summary of their atomic and physical properties Vanadium, niobium and tantalum have only the bcc, W-type, structure no high-temperature or high-pressure polymorphs are known. 

Notice that the structures presented in this paragraph are unary structures, that is one species only is present in all its atomic positions. In the prototypes listed (and in the chemically unary isostructural substances) this species is represented by a pure element. In a number of cases, however, more than one atomic species may be equally distributed in the various atomic positions. If each atomic site has the same probability of being occupied in a certain percentage by atoms X and Y and all the sites are compositionally equivalent, the unary prototype is still a valid structural reference. In this case, from a chemical point of view, the structure will correspond to a two-component phase. Notice that there can be many binary (or more complex) solid solution phases having for instance the Cu-type or the W-type structures. Such phases are formed in several metallic alloy systems either as terminal or intermediate phases. 

The body-centred cubic W-type structure. The W-type structure is another important structure of metallic elements it is common to a number of metals Li, Na, K, Rb, Ba, Cs, Eu, Cr, Mo, Y Ta, W, etc. (as the only room temperature stable form), Be, Ca, Sr, several rare earth elements, Th, etc. (as a high-temperature form) and a and 8 Fe forms. The data relevant to the prototype are reported in the following. 

Figure 7.12. Section sequence parallel to the base plane of the cI2-W type structure.

The homoleptic derivatives of Mo and W(VI) are rather scarcely studied. The only structurally characterized complex, W(OMe)5, possesses the molecular structure analogous to those of alkoxide halids, i.e. a dimer built up of two edge-sharing octahedra. The structure of monooxo homometallic derivatives is unknown and their individuality appears questionable. The only dioxocom-plex of molybdenum(V) isolated as pyridin solvate demonstrates the [Ti(OMe)w]-type structure (Table 12.19). 

Compounds isotypic with the k phases arc found among intcrmetallics, borides, carbides and oxides and also with silicides, germanides, arsenides, sulfides and sclcnides no nitrides, however, are found. The mode of filling the various voids in the metal host lattice of the k phases follows the schemein Ref. 4 and is presented in Table 1 for all those compounds for which the atom distribution is well known from x-ray or neutron diffraction. Accordingly, B atoms in tc-borides, Zr, Mo, W, Re)4B and Hfy(Mo, W, Re, Os)4B , occupy the trigonal prismatic interstices within the parent metal framework of a Mn, Aln,-type structure (see Table 1 see also ref. 48). Extended solid solutions are found for (Hf, Al)[Pg.140]

Gibson W (1996) Structure and assembly of the virion. Intervirology 39 389 00 Goldman ME, Nunberg JH, O Brien JA, Quintero JC, Schleif WA, Freund KF, Gaul SL, Saari WS, Wai IS, Hoffman JM et al. (1991) Pyridinone derivatives specific human immunodeficiency virus type 1 reverse transcriptase inhibitors with antiviral activity. Proc Natl Acad Sci USA... 

The reaction has been applied to the synthesis of cyclophane-type structures by use of dihaloarenes and Grignard reagents from a, w-dihalides. 

In the study by Johnson et al. (1986) it was shown by Hall effect measurements that the sheet carrier density was decreased and the mobility was increased for a thin w-type layer following exposure to a hydrogen plasma at 150°C. To explain the mobility increase it was argued that donor-H complexes were formed and that the concentration of ionized scattering centers was thereby decreased. On the basis of semiempirical calculations, a structural model was suggested for the donor-H complex in... 

Zinc oxide is normally an w-type semiconductor with a narrow stoichiometry range. For many years it was believed that this electronic behavior was due to the presence of Zn (Zn+) interstitials, but it is now apparent that the defect structure of this simple oxide is more complicated. The main point defects that can be considered to exist are vacancies, V0 and VZn, interstitials, Oj and Zn, and antisite defects, 0Zn and Zno-Each of these can show various charge states and can occupy several different... 

Sieghart, W. (1995) Structure and pharmacology of y-aminobutyric acidA receptor sub-types. Pharmacol. Rev. 47,181-234. 

A possible mechanism for such tight control is illustrated in Fig. 5. Clearly, increasing the protein concentration has a dramatic impact on the secondary structures of silk proteins in solution. The low concentration silk protein solution at 1% w/v is dominated by disordered structures or equally possible a polyproline II type structure (Sreerama and Woody, 2003). 

Meier, W.M. Olson, D.H. "Atlas of Zeolite Structure Types" Structure Commission of the Inti. Zeolite Assn., Polycrystal Book Service, Pittsburgh, PA, 1978. 

Figure 3.31. cF16-MnCu2Al type structure (representative of the Heusler type alloys). The unit cell is shown in (a), an eighth of the cell is shown in (b). It degenerates into a cP2-CsCl type cell if the atoms at the cube comers (Mn and Al) are equivalent. If all the atoms were equivalent there would he a further degeneration into the cI2-W type. 

W-N The so-called phase (—29.5 to 41.5 at.% N) is stable between 283°C and 1875°C and has the NaCl-type structure. WN (—50 at.% N) has the hP2-WC type structure. Several other phases, probably metastable, have been described. 

In the cP2-W type (CN 8) structure Vsph is 0.68 Vat (only a portion of the available space is occupied by the atomic sphere ). In the cF4-Cu type and in the ideal hP2-Mg type (CN 12) structures, Vsph is 0.74 Vat. Considering now the previously reported relationship between RCs n and i CN8, we may compute for a given element very little volume (Vat) change in the allotropic transformation from a form with CN 12 to the form with CN 8, because the radius variation is nearly... 

A traditional example of a Zintl phase is represented by NaTl which may be considered as a prototype of the Zintl rules. The structure of this compound (face centred cubic, cF16, a = 747.3 pm) can be described (see also 7.4.2.2.) as resulting from two interpenetrating diamond type lattices corresponding to the arrangements of the Na and T1 atoms respectively (Zintl and Dullenkopf 1932). Each T1 atom therefore is coordinated to other four T1 at a distance a)3/4 = 747.3)3/4 = 323.6pm which is shorter than that observed in elemental thallium (d = 341-346 pm in aTl, hP2-Mg type, CN = 6 + 6) and d = 336pm in /3 Tl, (cI2-W type, CN = 8). 

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