Intermetallic structures sections

Among the quaternary R-T-B-C compounds the R4T2B3C4 type being structurally related to the previously discussed R4T2B2C4-type structure (Section 2.3 n = 4) is, by far, found for a wider range of transition metals T than any other quaternary structure type in the R-T-B-C systems, suggesting that it represents a stable intermetallic structure type (Link et al., 2002). So far no superconductor has been found in the R4T2B3C4 family. 

In this group, the metals form compounds of the type Be M (M = Mg, Ca, Sr or Ba), MgjM (M = Ca, Sr or Ba) Mg4M or Mg23Mg (M = Sr or Ba) and MggM or Mg,7M2 (M = Sr or Ba) . These intermetallic phases are essentially identified and distinguished one from another by their x-ray diffraction patterns so their structures are included in this section. 

Our work described in this section clearly illustrates the importance of the nature of the cations (size, charges, electronegativities), electronegativity differences, electronic factors, and matrix effects in the structural preferences of polar intermetallics. Interplay of these crucial factors lead to important structural adaptations and deformations. We anticipate exploratory synthesis studies along the ZintI border will further result in the discovery of novel crystal structures and unique chemical bonding descriptions. 

Structural relations between quasicrystals and other intermetallic phases. As discussed in several sections of the review published by Kelton (1995) on quasicrystals and related structures, numerous studies and observations indicate structural similarities between non-periodic quasicrystal phases with crystalline phases and also, on the other hand, with amorphous, glassy and liquid phases. 

As mentioned above in the intermetallic Section, beta-tungsten, which is chemically WsO, is the prototype of the A-15 structure. The interest in WsO, or Ws01 x, is not only structural but is also based on the fact that this material is superconducting Further surprising is that, for the first time, this oxide superconductor has a higher transition temperature than that of the metal itself. Pure tungsten metal has a Tc of 15.4 mK, whereas the oxide WsO has a reported Tc of 3.35 K. Other oxide compounds such as CrsO and "MosO", which are isostructural with WsO, do not superconduct above 1.02 K. 

Most metals of practical importance are actually mixtures of two or more metals. Recall from Section 1.1.3 that these intimate mixtures of metals are called alloys, and when the bond between the metals is partially ionic, they are termed intermetallics. For the purposes of this chapter, and especially this section, we will not need to distinguish between an intermetallic and an alloy, except to note that when a compound is indicated on a phase diagram (e.g., CuAb), it indicates an intermetallic compound. We are concerned only with the thermodynamics that describe the intimate mixing of two species under equilibrium conditions. The factors affecting how two metal atoms mix has already been described in Section 1.1.3. Recall that the solubility of one element in another depends on the relative atomic radii, the electronegativity difference between the two elements, the similarity in crystal structures, and the valencies of the two elements. Thermodynamics does not yet allow us to translate these properties of atoms directly into free energies, but these factors are what contribute to the free energy of... 

This section differs from the previous one in that the materials considered here are obtained by combining two (or more) metals in well defined proportions corresponding to the appearance of a new phase. Since the electronic structure of the component metals is drastically changed in intermetallic compounds, it is expected that the mechanism of the hydrogen evolution reaction can change as well. The majority of... 

The most familiar metals are elemental substances such as iron, tin, aluminium etc. However, many compounds are metallic. As well as intermetallic compounds such as AgCd and NaTl, and a huge number of non-stoichiometric alloys, many oxides, sulphides, halides etc. have metallic properties. For details of structure and bonding in metallic substances, see Section 7.5. 

Some intermetallic compounds have the same structures as those of simple polar compounds. Quite a few AM type intermetallic compounds have the NaCl (3 2PO, Section 5.1.1) structure, but usually for those differing significantly in electronegativities. Table 5.1 includes many compounds of the types MP, MAs, MSb, and MBi. The NiAs structure (2-2PO, Section 5.2.1) is found for a few MAs and MSb compounds (Table 5.5), the MSn compounds of Fe, Ni, Cu, Pd, Pt, Rh, and Au, and MnBi, NiBi, PtBi, and RhBi. The ZnS structures (CN 4) are not usually encountered for intermetallic compounds. The compounds of Al, Ga, and In with P, As, and Sb have the zinc blende (ZnS, 3 2PT, Section 6.1.1) structure. These are semiconductors or insulators. Because the bcc structure is common for metals, it is not surprising that many 1 1 intermetallic compounds have the CsCl structure (3 2PTOT, Section 7.2.1). A few of these intermetallic compounds are included in Table 7.1 a more extensive list is given in Table 9.1. 

FeS2, 3 2PO). The structure of CaC2 (Section 5.1.2) is similar to that of pyrite, but with elongation in one direction because of alignment of C ions. Intermetallic compounds with the CaC2 structure are listed in Table 9.3. Thus intermetallic compounds can follow the rules for "normal valence" compounds or those of the metallic state. The "normal valence" compounds have lower CN than found in cep or hep structures and are expected to be less "metallic" in terms of electrical conductance, etc. 

The X-ray patterns taken from the two sections of the intermetallic layer adjacent to the Ni phase were also closely comparable, though this layer is visually seen in Fig. 3.13 to consist of three sublayers. One of these sections corresponded to a layer composition of 16.0 at.% Ni and 84.0 at.% Zn, while the other to 19.0 at.% Ni and 81.0 at.% Zn. The experimental interplanar distances were found to be in better agreement with the calculated values from the orthorhombic lattice parameters of =3.3326 nm, b=0.8869 nm and c=1.2499 nm reported by G. Nover and K. Schubert,279 rather than from the cubic lattice parameter =0.892 nm. Moreover, a few diffraction lines, including the strong line corresponding to the interplanar distance 0.207 nm, could not be indexed on the basis of the cubic structure. 

In the following, we proceed to several new aspects regarding molecular resolution of adsorbed metalloproteins. In Section 5.3, we first show that close to molecular resolution of the interfacial ET patterns of some two-centre proteins is within reach. The number of intermetallic interactions and resulting microscopic reduction potentials and rate constants in metalloproteins with more than two centres is, however, prohibitively large for such resolution. In Section 5.4 we address molecular and supramolecu-lar adsorption. We show here that in situ STM and AFM, indeed do hold exciting new perspectives for this important and central structural aspect of proteins at surfaces. 

As noted in section 6.2, when the material of interest is an intermetallic alloy, the solution of its crystal structure may be simplified because intermetallics often form series of isostructural compounds. In contrast to conventional inorganic and molecular compounds, stoichiometries of the majority of intermetallic phases are not restricted by normal valence and oxidation states of atoms and ions therefore, crystal structures of metallic alloy phases are conveniently coded using the classification suggested by W.B. Pearson. According to Pearson, each type of the crystal structure is assigned a specific code (symbol), which is constructed from three components as follows ... 

Between electron compounds and normal valence compounds, which are characterized by relatively small numbers of structures, there lie large groups of intermetallic compounds in which the structural principles are less clear. In the structures to be described in this section the importance of geometrical factors is becoming evident, a special feature of these structures being the high coordination numbers which range from 12 to 16. 

---