Nonstoichiometric hydrides

The prior treatment that the metal surface has received greatly affects the ease of hydride formation because the hydrogen must first be adsorbed on the metal surface before dissolution occurs. As a result, it is possible that not all of the available interstitial positions will become occupied by hydrogen, resulting in compositions that are variable depending on the temperature and pressure used in preparing the metal hydride. As a result, the composition may not be exactly stoichiometric, and hydrides of this type are sometimes referred to as nonstoichiometric hydrides. 

The properties of the nonstoichiometric hydrides are discussed in groups the saline hydrides, the Group IVA hydrides, the rare earth hydrides, the actinide hydrides, the Group VA hydrides, and palladium hydride. 

If with the specified values of c and T decomposition of nonstoichiometric hydride (IMC)if occurs in a-solid solution (IMC)ifa and H2, in the PCT-diagrams intervals of constant pressure (plateau) p( L) (T) appear in the (a+fi) double ... 

Although hydrogen can react with transition metals to form compounds such as UH3 and FeH6, most of the interstitial hydrides have variable compositions (often called nonstoichiometric compositions) with formulas such as LaH2.76 and VHo.56- The compositions of the nonstoichiometric hydrides vary with the length of exposure of the metal to hydrogen gas. 

Nonstoichiometric hydrides that have cubic CaFj-type structures or a tetragonally distorted version form when Ti, Zr and Hf react with Hj. The hydride phases have wide composition ranges that depend on T. The limiting composition is MHj. 

The invariant plateau pressure is the equilibrium absorption (dissociation) pressure of the hydride at the temperature of the isotherm. Reaction (2) is complete at point x and the hydrogen pressure again increases as the nonstoichiometric hydride absorbs hydrogen according to equation (3) ... 

The oxygen adsorption behavior of palladium has received far less attention than that of platinum and the main reason for this is the complication associated with the unusual capacity of palladium to adsorb large quantities of hydrogen. The metal forms a nonstoichiometric hydride of equilibrium composition PdHo69 at/>H2 = 1-0 25 C. Since the adsorption of hydrogen under... 

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