Metal hydride nonstoichiometric

Neutron diffraction studies have shown that in both systems Pd-H (17) and Ni-H (18) the hydrogen atoms during the process of hydride phase formation occupy octahedral positions inside the metal lattice. It is a process of ordering of the dissolved hydrogen in the a-solid solution leading to a hydride precipitation. In common with all other transition metal hydrides these also are of nonstoichiometric composition. As the respective atomic ratios of the components amount to approximately H/Me = 0.6, the hydrogen atoms thus occupy only some of the crystallographic positions available to them. 

When discussing metal alloys (Section 4.3), we saw that atoms of non-metallic elements such as H, B, C, and N can be inserted into the interstices (tetrahedral and octahedral holes) of a lattice of metal atoms to form metal-like compounds that are usually nonstoichiometric and have considerable technological importance. These interstitial compounds are commonly referred to as metal hydrides, borides, carbides, or nitrides, but the implication that they contain the anions H, B3, C4, or N3- is misleading. To clarify this point, we consider first the properties of truly ionic hydrides, carbides, and nitrides. 

The nature of the bonding in metallic hydrides is not well understood, and it s not known whether the hydrogens are present as neutral H atoms, H + cations, or H anions. Because the hydrogen atoms can fill a variable number of interstices, many metallic hydrides are nonstoichiometric compounds, meaning that their atomic composition can t be expressed as a ratio of small whole numbers. Examples are TiHj 7, ZrH19, and PdHv (x < 1). Other metallic hydrides, however, such as TiH2 and UH3, are stoichiometric compounds. 

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. 

Neutron and X-Ray Diffraction Studies of Nonstoichiometric Metal Hydrides... 

Metallic hydrides are usually nonstoichiometric compounds, as expected from their relatively low heats of formation and the mobility of hydrogen. They are ordinarily described, chemically, in terms of any of three models in which hydrogen is considered a small interstitial atom, a proton, or a hydride anion. These models are discussed critically with particular reference to the group V metal hydrides. The interstitial atom model is shown to be useful crystallographically, the protonic model is questioned, and the hydridic model is shown to be the most useful at present. The effect of hydrogen content on the lattice parameter of VHn and the structural and magnetic properties of several hydrides are discussed in terms of these models. 

Metallic hydrides are formed when hydrogen reacts with transition metals. These compounds are so named because they retain their metallic properties. They are not molecular substances, just as metals are not. In many metallic hydrides, the ratio of metal atoms to hydrogen atoms is not fixed or in small whole numbers. The composition can vary within a range, depending on reaction conditions. TH2 can be produced, for example, but preparations usually yield TiHi g. These nonstoichiometric metallic hydrides are sometimes called interstitial hydrides. Because hydrogen atoms are small enough to fit between the sites occupied by the metal atoms, many metal hydrides behave like interstitial alloys. — (Section 12.3)... 

Metallic hydrides are commonly formed with the transition elements— groups 3 to 12. A distinctive feature of these hydrides is that in many cases they are nonstoichiometric—the ratio of H atoms to metal atoms is variable, not fixed. This is because H atoms can enter the voids or holes among the metal atoms in a crystalline lattice and fill some but not others. 

Hydrides are compounds of hydrogen, usually divided into the categories of covalent (e.g., H2O and HCl), ionic (e.g., LiH and CaH2), and metallic (mostly nonstoichiometric compounds with the transition metals). 

The binaiy hydrides (p. 64), borides (p. 145), carbides (p. 299) and nitrides (p. 417) are hard, refractory, nonstoichiometric materials with metallic conductivities. They have already been discussed in relation to comparable compounds of other metals in earlier chapters. 

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. 

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