Thermodynamics of Metal Hydrides

The above division should not be taken too literally. In fact, most of the metal hydrides have a mixture of different types of bonding. Eor example, in LiH the bonds are mainly ionic but still have a significant covalent component [35]. 

As most of the hydrides discussed here are formed by direct reaction with gaseous hydrogen, it is important first to discuss the thermodynamics of hydride formation. In this section, the thermodynamic aspect of hydride formation will be briefly sketched. In-depth treatment of the thermodynamics of metal-hydrogen systems can be found in the literature [32, 40-44]. 

When exposed to hydrogen, a hydride-forming metal (M) will form a metal hydride following the reaction  

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  

As the hydrogen pressure increases, the concentration also increases until the attractive H-H interaction becomes important [32]. At this point, nucleation of a higher concentration phase ((3 phase) occurs. The system now has three phases (a, (3 and hydrogen gas) and two components (metal and hydrogen). From the Gibbs phase rule, the degree of freedom (/) is  

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