Electron structure and thermodynamics of solid solutions in Ni–H system

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 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 observed in the hydrogen–charged nickel with H/Ni ratio of about 0.7. 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 Gibb’s types of precipitation reactions. Keywords: nickel, hydrogen, ab initio calculations, spinodal decomposition, hydride