Thermodynamics of Ni-H System based on the Atomistic Calculations Combined with Measurements of X-ray Diffraction

The primary aim of the present study was to clarify thermodynamics of Ni-H system based on the atomistic calculations combined with measurements of X-ray diffraction and discuss the experimental data in terms of two possible precipitation reactions in solid solutions. The widespread concept of nickel hydride in the Ni-H system is discussed based on the first-principle atomic calculations and experimental X-ray diffraction data. The total cohesion energy in Ni-H solid solution has been determined using the density functional theory and program package Wien2k. Its dependence on hydrogen concentration is shown to be linear, which suggests the absence of any energy barrier for precipitation reaction. Moreover, the second derivative of the calculated solution enthalpy is negative within the hydrogen-to-nickel ratios, H/Ni, of 0.03 to 0.75, which is a sign of spinodal decomposition. These hydrogen concentrations are consistent with the measurements of X-ray diffraction, of which the results are traditionally interpreted in terms of Ni hydride. The density of electron states has been calculated, and its non-monotonous concentration dependence correlates with that of solution enthalpy, which is also expected for spinodal decomposition. The obtained results are interpreted as miscibility gap in the Ni-H system with spinodal decomposition having the electron origin. In addition, using mechanical spectroscopy, the strain dependent internal friction has been measured in the hydrogen-charged nickel in the comparison with that in titanium. This effect is controlled by irreversible plastic deformation, which is typical for solid solutions, not for brittle chemical compounds. Finally, the “hydrides” in a number of metals are discussed in terms of two Gibbs types of precipitation reactions. The analysis of a number of studies on “metal hydrides” apparently obtained under high gaseous hydrogen pressures leads to the conclusion that, in the majority, the formation of oversaturated solid solutions or the γ−→ξ phase transformation between two solid solutions occurs.