Intermetallic compound crystal type

Introduction. A number of common structures, ideally corresponding to a 1 1 stoichiometry, are presented in this chapter. Some of them are not specifically characteristic of intermetallic compounds only. The CsCl and NaCl types, for instance, are observed for several kinds of chemical compounds (from typical ionic to metallic phases). Notice that for a number of prototypes a few derivative structures have also been considered and described, underlining crystal analogies and relationships even if with a change in the reference stoichiometry. 

Well-ordered intermetallic compounds (alloys), when processed in high-energy ball mills exhibit atomic (chemical) disordering in the early stages of ball milling [153]. Let us take as an example ordered AlRu intermetallic crystals that are of B2 type and P-CuZn structure. This structure consists of two simple cubic interpenetrating... 

It is crucial to discover the relationship between chemical compositions and hydrogen decomposition pressures of the AB5 compounds. Intermetallic compounds of lanthanide and transition metals form an interesting class of structures. The AB5 series crystallize in the hexagonal CaCus (P6/mmm) structure (see Figure 1). Generally, radius ratios (raAb) greater than 1.30 form the CaCus-type... 

In pure titanium, the crystal structure is dose-packed hexagonal (a) up to 882°C and body-centered cubic (p) to the melting point. The addition of alloying dements alters the a—p transformation temperature. Elements that raise the transformation temperature are called a-stabilizers those that depress the transformation temperature, p-stabilizers the latter are divided into p-isomorphous and p-eutectoid types. The p-isomorphous elements have limited a-solubility and increasing additions of these dements progressively depresses the transformation temperature. The p-eutectoid elements have restricted p-solubility and form intermetallic compounds by eutectoid decomposition of the p-phase. The binary phase diagram illustrating these three types of alloy... 

In some cases, empirical rules can also relate thermodynamic properties to crystal structures. One of the best-known cases is in the AB5 systems where the equilibrium pressure is linearly correlated to the cell volume. As the cell volume increases, the equilibrium plateau pressure decreases, following a InPn law [64]. However, some exceptions exist to this rule such as in LaPts where electronic effects make the smaller unit cell more stable [65[. Nevertheless, generally, for intermetallic compounds the stability of the hydride increases with the size of the interstices [66]. A limitation of this empirical rule is that comparison between different types of intermetallics is impossible. For example, the stabilities of AB2 alloys cannot be compared with those of AB5 alloys [43]. 

In the case of semiconductors, it was first shown in this laboratory that the arrangements of the atoms in the surface monolayers of (100) and (111) germanium and silicon are not the same as those for these planes in the bulk (25). The altered arrangements were revealed by the presence of fractional order beams for the surface gratings in certain azimuths. This was later found to be the case for all crystals tested which have a diamond-type lattice, including semiconducting diamond and several of the intermetallic compounds. The surface structure of silicon was observed to be much more complex than that of germanium. In some azimuths, several fractional orders less than one-half were observed. 

Some intermetallic compounds having the diamond or zinc blende structures, such as GaSb and InSb show an altered surface structure (19) as indicated by the presence of fractional order diffraction beams after cleaning by the argon-ion bombardment and annealing technique. For crystals of this type there is asymmetry in opposite directions perpendicular to (111) planes. This asymmetry results in atoms of type A on one surface and those of type B on the opposite surface (20). The possibility of detecting effects of the asymmetry of clean surfaces by electron diffraction has been considered. However, in the cases of GaSb and InSb, no difference in the diffraction patterns from these two surfaces has been detected. 

Since the X-ray diffraction studies of Zintl et al. , these members of the family of intermetallic compounds have been of special interest because some of their chemical properties are unusual for intermetallic phases. Many experimental investigations have been reported for binary and ternary B32 type compounds. Besides the crystal structure " , the thermodynamic behavior , electrical conductivity ", magnetic susceptibility , NMR data elastic constants - and optical properties have been studied. Additionally for LiAl electrochemical investigations have been performed in view of the recent interest in fast ionic conductors " . ... 

In Sect. C, the band structure data based on self-consistent relativistic augmented-plane-wave calculations performed by the author " are presented. Besides the electronic bands and the densities of states, the nature of the chemical bond is discussed. In Sect. D the electronic states in Zintl phases are compared with those having the B2 type of structure. As shown in Sect. B the B2 structure is closely related to the B32 structure. For intermetallic compounds the B2 structure seems to be the more natural because in this lattice all nearest neighbours of an atom A are B atoms. The reason why the compounds mentioned above crystallize in the B32 structure whereas similar compounds like LiTl and KTl form B2 phases has been frequently discussed in the literature 5 ... 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 0,m in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fine oxide particles can be prepared from metals, alloys, and intermetallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-methoxyethanol at 473-523 K yields ultrafine ceria particles (ca. 2 nm). 

Wo have already made the general statement that when any two metals are used to form an alloy the actual structure of the alloy depends on whether, for the medals concerned, the tendency to produce a solid solution is greater than the opposing tendency to form an intermetallic compound. Since the formation of solid solutions is mainly governed by the relative sizes of the two atoms, while compound formation involves relationships between the atoms and their extra-nuclear electrons, it is probably more convenient to discuss first the sizes of metal atoms, usually expressed in terms of atom diameters. We have recently shown that in all types of pure metal crystals there are certain directions of atom close packing, and if, as is convenient, we continue to regard atoms as spheres, it is readily seen that the shortest interatomic distances are of the same magnitude as the respective atomic diameters. We shall therefore proceed to discuss the closest distance of approach of atoms in the typical metallic lattices. 

Solid solutions can form in metals if the atoms of which they are composed.are similar also, compounds can form. In such cases, expressions for the excess Gibbs energy of solid mixtures should contain a strain or mechanical energy term (which results from distorting the crystal structure to accommodate an atom of different size), a valence or coiilombic term to account for the difference in charge between the solute atom and the atoms of the host crystal, the noncoulombic interactions of the type we considered in discussing molecular fluids in Sec. 9.5, and perhaps a chemical reaction term to account for compound formation. Alloys, amalgams, and intermetallic compounds can occur in solids these more complicated situations will not be considered here. 

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