Metal lattice, hosts

In contrast to the interstitial hydrides, where the metal lattice hosts the hydrogen atoms on interstitial sites, the desorption of the hydrogen from the complex hydride leads to a complete decomposition of the complex hydride and a mixture of at least two phases is formed. For alkali metal tetrahydroborates and tetrahydroaluminates, the decomposition reaction is described according to the following equation ... 

The hydrogen is, at small hydrogen to metal ratios (H M<0.1), exothermically dissolved (solid solution, a-phase) in the metal. The metal lattice expands proportionally to the hydrogen concentration by approximately 2-3 A per hydrogen atom [31]. At greater hydrogen concentrations in the host metal (H M > 0.1), a strong... 

H-H interaction due to the lattice expansion becomes important and the hydride phase (P phase) nucleates and grows. The hydrogen concentration in the hydride phase is often found to be H M = 1. The volume expansion between the coexisting a- and P-phases corresponds in many cases to 10-20% ofthe metal lattice. Therefore, at the phase boundary high stress is built up and often leads to decrepitation of brittle host metals such as intermetaiiic compounds. The final hydride is a powder with a typical particle size of 10-100 pm (Figure 5.24). 

In interstitial compounds, however, the nonmetal is conveniently regarded as neutral atoms inserted into the interstices of the expanded lattice of the elemental metal. Obviously, this is an oversimplification, as the electrons of the nonmetal atoms must interact with the modified valence and conduction bands of the metal host, but this crude picture is adequate for our purposes. On this basis, Hagg made the empirical observation that insertion is possible when the atomic radius of the nonmetal is not greater than 0.59 times the atomic radius of the host metal—there is no simple geometrical justification for this, however, as the metal lattice is concomitantly expanded by an unknown amount. These interstitial compounds are sometimes called Hagg compounds.9,10 They are, in effect, interstitial solid solutions of the nonmetal in the metal (as distinct from substitutional solid solutions, in which actual lattice atoms are replaced, as in the case of gold-copper and other alloys Section 4.3). 

Cao, G., L. K. Rabenberg, C. M. Nunn, and T. E. Mallouk. 1991. Formation of quantum-size semiconductor particles in a layered metal phosphonate host lattice. Chem. Mater. 3 149-156. 

At low concentration (x 1) hydrogen first dissolves in the metal lattice and forms a solid solution phase (a phase). Hydrogen is then randomly distributed in the metal host lattice and the concentration varies slowly with temperature. The a phase has the same crystal structure as the bare metal. The condition for thermodynamic equilibrium is given by ... 

Thermodynamically, it is possible for metallic Mo to be codeposited in a host lattice provided its Gibbs energy is thereby sufficiently lowered. An analogous situation is the deposition of Na into Hg at some 1.2 V less negative than its normal (standard) electrode potential. The extensive coevolution of Hj during Mo electrodeposition with Ni or Co indicates that the overall process of alloy or composite metal deposition is far from efficient and that electrosorbed H may easily also be codeposited into the joint metal lattice, providing a hydride phase. 

There are two types of alloys. In a substitutional alloy, some of the metal atoms in a crystal lattice are replaced by other atoms (usually of comparable size). Examples are brass, in which approximately one third of the atoms in a copper crystal are replaced by zinc atoms, and pewter, an alloy of tin that contains 7% copper, 6% bismuth, and 2% antimony. In an interstitial alloy, atoms of one or more additional elements enter the interstitial sites of the host metal lattice. An example is steel, in which carbon atoms occupy interstitial sites of an iron crystal, making the material stronger and harder than pure iron. Mild steel contains less than 0.2% C and is used for nails, whereas high-carbon steels can contain up to 1.5% C and are used in specialty applications such as tools and springs. Alloy steels are both substitutional and interstitial atoms from metals such as chromium and vanadium substitute for iron atoms, with carbon remaining in interstitial sites. Alloy steels have a variety of specialized purposes, ranging from cutlery to bicycle frames. 

One of the important applications of butylcalix[4]arenes arises from their ability to trap alkali metal ions. In particular, Cs+-calixarene complexes have received much attention because of the need to remove the Cs radionucleotide from nuclear wastes. Benevelli et al. have used one-pulse solid state NMR experiments to directly observe Li, Na and Cs ions in the host cavity [52]. More advanced experiments, which allow the investigation of metal lattice interactions were also reported. Rotational-echo double resonance (REDOR) NMR is a useful tool for obtaining structural details of butylcalix[4]arene [53]. Gullion and coworkers used REDOR to determine the position of the... 

The electrochemical reaction leads to a more reduced or more oxidized state in the product than in the molten salt. Only electroreduction has so far been used to prepare compounds with tunnel structures. In such structures the tunnels contain ions, e.g., alkali-metal ions in a transition-metal oxide host lattice. Ionization of the inserted alkali-metal gives electrons to the oxide host, reducing the transition-metal cations. 

Core-polarisation and conduction-electron polarisation effects can be studied as can exchange polarisation of diamagnetic atoms in magnetic hosts. The lattice dynamics of the metal lattice are examined via the temperature dependence of the /-factor. Many metals approximate closely to the Debye model, and a Debye temperature has some significance. Impurity doping can... 

Fig. 5 Typical examples of (a-c) self-assembling inclusion hosts (d-f) interlocked and interwoven systems and (g-j) solid state inclusion hosts (a) tennis ball dimer (b) metallamacrocycle (c) ladder-structured metal array (d) catenane (e) rotaxane (f) helicate (g) network lattice host (exemplary host molecule) (h) coorclinatoclathrate host (exemplary hosts) (i) curved framework host molecule and (j) aukward-shape host molecule.

An interstitial structure is one in wiiich the ions or atoms of a ncHimetallie element, typically carbon for carbides, nitrogen for nitrides, or hydrogen for hydrides, occupy certain interstitial sites within a metal lattice. Expressed in geometrical terms, the ratio of the radius of the interstitial atom to the radius of the atom of the host metal must be less than 0.59 for an interstitial structure to be formed. 1 1... 

The structural diversity of these materials is so great as to preclude a full discussion here, but we can conveniently consider them in terms of the categories shown in Table 13.3, which are identified in terms of the arrangement of the B atoms within a host metal lattice. The structure of the MBg borides (e.g. CaBg) is similar to a CsCl-type structure with Bg-units (Table 13.3) replacing... 

Recently three-dimensional metal complex hosts have been developed from the two-dimensional Hofmann type host lattices, M(NH3)2Ni(CN)4, by replacing the ammonia groups by bidentate ligands, with the aim of enlarging the range of guest molecules which can be accommodated in the host lattices [1-5]. In a previous study Mathey et al. reported the preparation of the Ni(4,4 -bipyridyl)Ni(CN)4 host lattice and its benzene, xylene, naphthalene and anthracene clathrates [5]. We have extended this study and prepared M(4,4 -bipyridyl)Ni(CN)4-2G (M = Ni or Cd G = dioxane, toluene, aniline or iV,AT-dimethylaniline) clathrates for the first time. In this study an IR spectroscopic study of the M(4,4 -bipy)Ni(CN)4 -MG compounds (where M = Ni or Cd, G = dioxane, benzene, toluene, aniline or iV,AT-dimethylaniline, n=0-2) (abbreviated henceforth as M-Ni-bipy-G) are reported. Additional information is obtained from the laser-Raman spectrum of the Cd-Ni-bipy complex. We also recorded the powder X-ray diffraction patterns of the M-Ni-bipy complexes. 

The rare earths absorb hydrogen readily and form solid solutions and/or hydrides exothermally at temperatures of several hundred C. Their phase diagrams consist, in general, of three basic parts (a) the metallic solid solution, or a-phase, with the H atoms inserted in the tetrahedral interstices of the host-metal lattice (b) the equally metallic dihydride 3-phase, where the two H atoms occupy ideally the two available tetrahedral sites this phase crystallizes in the fee fluorite system (c) the insulating trihydride, or y-phase, which possesses an hep unit cell with both tetrahedral sites and the one octahedral site filled up. A schematic view is given in fig. 1. Exceptions are the divalent lanthanides Eu and Yb, whose dihydrides are already insulators and exhibit an orthorhombic structure, and Sc whose very small unit cell does not normally accept more than two H atoms. 

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