Hydrides, binary interstitial

Generally chemists recognize three broad types of binary hydrides (that is, compounds containing hydrogen and one additional element). These are molecular hydrides, saltlike hydrides, and interstitial hydrides. 

According to the rule of imaginary binary hydrides, the stability of hydrogen on an interstitial site is the weighted average of the stability of the corresponding binary hydrides of the neighboring metallic atoms [35]. 

Hydrogen forms three types of binary hydrides. Active metals give ionic hydrides, such as LiH and CaFF nonmetals give covalent hydrides, such as NH3, H2O, and HF and transition metals give metallic, or interstitial, hydrides, such as PdH,.. Interstitial hydrides are often nonstoichiometric compounds. 

At that date, palladium hydride was regarded as a special case. Lacher s approach was subsequently developed by the author (1946) (I) and by Rees (1954) (34) into attempts to frame a general theory of the nature and existence of solid compounds. The one model starts with the idea of the crystal of a binary compound, of perfect stoichiometric composition, but with intrinsic lattice disorder —e.g., of Frenkel type. As the stoichiometry adjusts itself to higher or lower partial pressures of one or other component, by incorporating cation vacancies or interstitial cations, the relevant feature is the interaction of point defects located on adjacent sites. These interactions contribute to the partition function of the crystal and set a maximum attainable concentration of each type of defect. Conjugate with the maximum concentration of, for example, cation vacancies, Nh 9 and fixed by the intrinsic lattice disorder, is a minimum concentration of interstitials, N. The difference, Nh — Ni, measures the nonstoichiometry at the nonmetal-rich phase limit. The metal-rich limit is similarly determined by the maximum attainable concentration of interstitials. With the maximum concentrations of defects, so defined, may be compared the intrinsic disorder in the stoichiometric crystals, and from the several energies concerned there can be specified the conditions under which the stoichiometric crystal lies outside the stability limits. 

Some solid-state metal hydrides are commercially (and in some cases potentially) very important because they are a safe and efficient way to store highly flammable hydrogen gas (for example, in nickel-metal hydride (NiMH) batteries). However, from a structural and theoretical point of view many aspects of metal-hydrogen bonding are still not well understood, and it is hoped that the accurate analysis of H positions in the various interstitial sites of the previously described covalent, molecular metal hydride cluster complexes will serve as models for H atoms in binary or more complex solid state hydride systems. For example, we can speculate that the octahedral cavities are more spacious in which H atoms can rattle around , while tetrahedral sites have less space and may even have to experience some expansion to accommodate a H atom. 

Hydride a binary compound containing hydrogen. The hydride ion, H, exists in ionic hydrides. The three classes of hydrides are covalent, interstitial, and ionic. (18.3) Hydrocarbon a compound composed of carbon and hydrogen. (22.1)... 

Binary hydrides are compounds containing hydrogen and another element, either a metal or a nonmetal. Depending on strnctnre and properties, these hydrides are broadly divided into three types (1) ionic hydrides, (2) covalent hydrides, and (3) interstitial hydrides. 

In Chapters 20 and 21 we shall look at individual elements of the c -block in detail. However, a few general points are given here as an overview. In general, the metals are moderately reactive and combine to give binary compounds when heated with dioxygen, sulfur or the halogens (e.g. reactions 19.1-19.3), product stoichiometry depending, in part, on the available oxidation states (see below). Combination with H2, B, C or N2 may lead to interstitial hydrides Section 9.7), borides Section 12.10), carbides Section 13.7) or nitrides Section 14.6). 

Whether it is possible for a tetrahedral cluster to accommodate an H atom in its interstitial cavity without causing substantial swelling and hence destabilization of the cluster is an intriguing question. Although the occupation of tetrahedral holes is a well-docixmented phenomenon in binary metal hydrides where the metallic lattice... 

What criterion deddes whether an element is able to contribute as an interstitial atom to the stabilization of an electron defident duster Obviously, the M6 octahedron bears some relation to a microscopically small piece of metal. The vibrational frequent of the H atom in the [NbgH] unit of Nb(I,iH is nearly identical to the frequency of H in the metallic hydride NbH. [92] Since the bonding in the duster and that in the extended metal lattice are so similar, the obvious question to ask is which elements form stable compounds with the bulk metal that represents the duster atoms. The answer to this question yields a qualitative explanation for the fact that the electron defident Nb unit incorporates H, while the Mog octahedron does not. Zr forms numerous intermetallic compounds with Be, Al, and other d metals and, obviously, does not loose this ability when only six Zr atoms are joined. Inspection of the experimental data for the relevant binary systems or the use of Miedema s concept for the stability of intermetallic systems [93, 94] proves helpful in the search for possible interstitial atoms, and naturally limiting the search to atoms of appropriate size to fit into the octahedral site. Of course, if the intermetallic compounds are very stable they could also compete with the duster compound formation. 

---