The crystal structure and some thermodynamic properties of the - hydrogen system

Abstract

A structural study of the alloys revealed that nickel in can be replaced by aluminium up to the composition . However, the hexagonal crystal structure of the binary compound ( type, space group P6/mmm) exists only up to , while beyond this composition another hexagonal structure ( type, space group P6/mmm) is stable. All of the single-phase alloys have been exposed to hydrogen gas at pressures up to 15 MPa and temperatures between 77 K and 700 K. It was found that under the above conditions only ternary alloys having the structure react with hydrogen, whereby the hydrogen uptake increases the unit-cell volume by up to 12.5%. It was also found that the hydrogen equilibrium pressures decrease with the increasing content of aluminium as does the hydrogen capacity at room temperature. Binary , and the ternary alloys crystallizing with the type of structure, do not show any significant hydrogen absorption. The entropy, the enthalpy, and the Gibbs free energy of formation have been extracted for the systems that exhibit an equilibrium plateau in the pressure - composition isotherms.