Tuning Stability of AB3-Type Alloys by Suppressing Magnetism

TL;DR

This work tackles the magnetism–stability trade-off in AB3-type hydrogen storage intermetallics. Using a multiscale framework that combines first-principles CPA, Liechtenstein exchange calculations, and Monte Carlo simulations, it shows a direct correlation between formation energy and total magnetic moment in Co-based Ca_xY_yMg_1-x-yCo3, with magnetism stabilizing the lattice in low-volume cases and destabilizing it in high-volume ones. Replacing Co with Ni suppresses magnetism (YNi3 is nonmagnetic; CaNi3 and MgNi3 are weakly magnetic), enabling thermodynamic stability across compositions and expanding high-capacity regions; notable instances include CaMg2Ni9 at ~3.32 wt% and Mg-rich Ni-based alloys approaching ~3.40 wt% gravimetric density. Overall, the paper establishes magnetism as a design lever to reconcile stability and capacity in AB3 hydrides and suggests magnetic control as a general strategy for related intermetallic systems.

Abstract

Hydrogen is a promising clean energy carrier, yet effective and reversible storage remains challenging. AB3-type intermetallic alloys are promising for solid-state hydrogen storage due to intermediate thermodynamic stability and rapid hydrogen uptake. Optimizing stability and gravimetric density is hindered by competing thermodynamic and magnetic effects. Here, we analyze AB3 compounds (A = Ca, Y, Mg; B = Co, Ni) and ternary alloys CaxYyMg1-x-yB3 using first-principles calculations and Monte Carlo simulations. We find a direct correlation between formation energy and total magnetic moment that dictates alloy stability, explaining the trade-off in hydrogen storage. In Co-rich systems with large lattice volumes, formation energy rises with magnetization, showing magnetism as the dominant factor. Mg-rich compositions achieve high gravimetric densities, but strong magnetism destabilizes the system, requiring Y substitution to suppress magnetic moments. Replacing Co with Ni weakens magnetism: YNi3 is nonmagnetic, while CaNi3 and MgNi3 are weakly polarized, allowing thermodynamic stability across compositions. Notably, CaMg2Ni9 combines high theoretical capacity (3.32 wt%) with good reversibility. Mg-rich Ni-based alloys are predicted to offer negative formation energies with the highest gravimetric densities (up to 3.40 wt%). These results show that controlling magnetism via transition-metal substitution is key to overcoming the stability-capacity trade-off in AB3 hydrogen storage materials.

Figures (10)

• Figure 1: Crystal structure of stoichiometric AB$_{3}$ compounds (A = Ca, Y, Mg; B = Co, Ni) with rhombohedral space group $R\bar{3}m$ (No. 166). The structure can be conceptualized as a combination of AB$_{2}$ structural units (highlighted in orange) and AB$_{5}$ structural units (highlighted in green), representing two-thirds and one-third contributions, respectively. Large orange spheres represent A-site atoms (Ca, Y, or Mg) occupying both 3$a$ (AB$_{5}$ unit) and 6$c$ (AB$_{2}$ unit) Wyckoff positions, while smaller green spheres represent B-site atoms (Co or Ni).
• Figure 2: Formation energy diagrams of Ca$_{x}$Y$_{y}$Mg$_{1-x-y}$Co$_{3}$ calculated using Eq. \ref{['Eq1']} with the crystal structures of CaCo$_{3}$ (a), YCo$_{3}$ (b), and MgCo$_{3}$ (c). Corresponding total magnetic moment diagrams of Ca$_{x}$Y$_{y}$Mg$_{1-x-y}$Co$_{3}$ with the crystal structures of CaCo$_{3}$ (d), YCo$_{3}$ (e), and MgCo$_{3}$ (f). Formation energies are given in eV per formula unit, and magnetic moments in $\mu_{\mathrm{B}}$ per formula unit. Black squares indicate compounds reported in previous experimental studiesLiu2003JACSato2025JPCC.
• Figure 3: Average formation energy (a) and total magnetic moment (b) diagrams of Ca$_{x}$Y$_{y}$Mg$_{1-x-y}$Co$_{3}$ calculated using Eq. \ref{['Eq2']}. (c) Theoretical gravimetric hydrogen density diagram of Ca$_{x}$Y$_{y}$Mg$_{1-x-y}$Co$_{3}$H$_{7}$ assuming complete hydrogenation from Eq. \ref{['Eq3']}. Formation energies are given in eV per formula unit, magnetic moments in $\mu_{\mathrm{B}}$ per formula unit, and hydrogen densities in weight percent. Black squares indicate compounds reported in previous experimental studiesLiu2003JACSato2025JPCC. (d) Formation energy versus total magnetic moment of Ca$_{x}$Y$_{y}$Mg$_{1-x-y}$Co$_{3}$ with CaCo$_{3}$ (light-red), YCo$_{3}$ (light-blue), and MgCo$_{3}$ (light-green) structures. The average formation energy versus average total magnetic moment (black) is estimated by using Eq. \ref{['Eq2']}.
• Figure 4: Total and partial density of states (DOS) of CaCo$_{3}$ (a), YCo$_{3}$ (b), and MgCo$_{3}$ (c). The total DOS is shown as light-red curves, while the partial DOS of A atoms (Ca, Y, Mg) and Co atoms are shown as light-blue and light-green curves, respectively. The partial DOS corresponds to the sum over all atoms of the same type in the unit cell. Magnetic exchange coupling constants of CaCo$_{3}$ (d), YCo$_{3}$ (e), and MgCo$_{3}$ (f), where A--A, A--Co, and Co--Co pairs are denoted by light-green triangles, light-blue squares, and light-red circles, respectively. Spin tends to parallel (P) in the case of $J_{ij} > 0$, while $J_{ij} < 0$ favors antiparallel (AP).
• Figure 5: Temperature dependence of (a) magnetization and magnetic susceptibility, and (b) magnetic energy for CaCo$_{3}$, YCo$_{3}$, and MgCo$_{3}$. Magnetization and magnetic energy are shown as solid curves in light-red, light-blue, and light-green for CaCo$_{3}$, YCo$_{3}$, and MgCo$_{3}$, respectively. Magnetic susceptibility is shown as dashed curves with the same color scheme.