The application of Pettifor structure maps to binary metal hydrides

Abstract

The aim of this paper is the application of Pettifor stability structure maps to MHx binary metal (M) hydrides in accordance with their hydrogenation properties. The formation enthalpy (ΔHf) of selected hydrides, their stoichiometry (MHx) and hydrogen capacity (Hcap) were extracted from the literature, where these values were either measured or calculated using various models. A comparison of the enthalpy values obtained from experiments or calculations based on the Miedema, Born–Haber and Energetic models is also presented. An assessment of the effect of the ΔHf values obtained by various methods on the structure stability maps was performed by plotting the enthalpy vs. the Mendeleev number M and the value of the chemical scale, χ. Moreover, based on two- and three-dimensional Pettifor plots, the enthalpy of hydride formation and hydrogen capacity of selected hydrides are presented with respect to the hydride stoichiometry, Hx, and the Mendeleev number M. Higher values of M indicate less hydride stability and, thus, larger stability variations. The metal hydrides with only one hydrogen atom (x = 1) and M from 0 to 21 exhibit stability that is adequate for practical applications. For M from 21 to 53, all hydrides exhibit high stability regardless of Hx. For M from 53 to 102, all hydrides exhibit large variations in stability. Thus, the hydrides with x = 1–3 exhibit either high stability or stability that is adequate for practical applications, and those with x = 4 are highly unstable hydrides.