Solution of hydrogen in metals

Endothermic occlusion takes place by diffusion of hydrogen into a metal lattice which is very little changed by the process. In exothermic occlusion by palladium, however, the face-centred cubic lattice (a phase) of palladium, with lattice constant 3.88 A, will accomodate, below 100°C, no more than about 5 at. % hydrogen, and then undergoes a transition to an expanded phase ( 3 phase), with lattice constant 4.02 A and H/Pd = 0.5—0.6. The H—Pd system thus splits into a and 3 phases in the manner familiar for two partially miscible liquids. The consolute temperature (rarely observable for solid phases) is about 310°C at H/Pd = 0.22. The phase diagram is, however, not well established because formation of the 

---

The body-centered cubic Group VA metals dissolve a considerable amount of hydrogen before distorting to either a b.c. tetragonal or orthorhombic structure. These behave somewhat more like solid solutions of hydrogen in metal than do the more definite hydrides previously discussed. They appear to have the stoichiometric formula MH, but this has never been reported, the maximum being about... 

Strictly a-solid solution of hydrogen in a metal catalyst studied. 

The slope is, for a specific system, given by the enthalpy and entropy of solution of hydrogen in the metal in other words by the energetics of the following reaction... 

Values for the enthalpy of solution of hydrogen in transition metals at infinite dilution shown in Figure 7.22 are more negative for the early transition metals. It should be noted that the enthalpies of solution in general are functions of the concentration of the solute. Still, the values at infinite dilution are useful when looking for systematic variations, particularly since changes with composition are often limited. 

Figure 7.22 Enthalpy of solution of hydrogen in transition metals at infinite dilution [45].

The transition metal hydrides exhibit such wide variations from stoichiometric compositions that they have often been considered interstitial solid solutions of hydrogen in the metal. This implies that the metal lattice has the same structure in the hydride phase as in the pure metal. That this is not the case can be seen in Table I, where of 28 hydrides formed by direct reaction of metal and hydrogen, only three (Ce, Ac, Pd) do not change structure on hydride formation. Even in these three cases, there is a large discontinuous increase in lattice parameter. The change in structure on addition of hydrogen plus the high heats of formation (20 to 50 kcal. per mole) (27) indicates that the transition metal hydrides should be considered definite chemical compounds rather than interstitial solid solutions,... 

Almost thirty years before the date of Moissan s work, a very unstable, crystalline, silver-white compound was described by Troost and Hautefeuille.2 They obtained it by sorption of hydrogen by means of sodium above 300° C., the metal sorbing 237 times its volume of the gas. That their product was a definite chemical compound is doubtful it was probably a solid solution of hydrogen in sodium, analogous to that formed by palladium. 

The preparation of TcH occurs at 300°C and 1.9 GPa Hj with the composition TcHg j. Both TcHp,3 and the hydrogen-free metal have an hep metal lattice, which makes it difficult, without extensive x-ray work showing the dependence of lattice parameters on composition, to determine whether TcHq is a hydride phase or just a solid solution of hydrogen in Tc. However, from the behavior of the electrical resistance of TcH as a function of composition, it can be deducted that a hydride phase is formed below 300°C. 

Hydrogen reacts with metal borides, carbides, silicides, nitrides, phosphides, oxides, sulfides, and halides to form a solid solution of hydrogen in the compound with... 

A solid solution of hydrogen in technetium metal is formed during the electrodcpo-sition of technetium from acid aqueous solutions of pcrtechnctate. The maximum hydrogen content of the amorphous phase corresponded to TcHo.27 [6]. 

Metallic hydrides are formed by the transition metals. Transformation of a pure metal into a hydride occurs through continuous solution of hydrogen in the metal with subsequent abrupt phase transition at defined stoichiometric hydride phases (ZrH2, PdH. VH, VH2). 

The pressure of hydrogen in equilibrium with the saturated solution of hydrogen in the metal is given by ... 

Most of the data for the solubility of hydrogen in metals are based on equilibrium with hydrogen gas. At moderate pressures, the lattice concentration of dissolved hydrogen obeys Sievert s law that is, the hydrogen concentration is proportional to the square root of the pressure. For iron and other low solubility elements, the concentration increases exponentially with temperature. The heat of solution is positive for endothermic absorbers, with a value of 27.2 to 28.6 kj mol in the case of iron [38-40]. For iron, the lattice solubility, Cs, of hydrogen in equilibrium with hydrogen gas can be expressed as [39]... 

---