Reaction of hydrogen with the low-temperature form (C15) of titanium-chromium (TiCr2)

On-the-Fly Active Learning of Interatomic Potentials for Large-Scale Atomistic Simulations

The on-the-fly generation of machine-learning force fields by active-learning schemes attracts a great deal of attention in the community of atomistic simulations. The algorithms allow the machine to self-learn an interatomic potential and construct machine-learned models on the fly during simulations. State-of-the-art query strategies allow the machine to judge whether new structures are out of the training data set or not. Only when the machine judges the necessity of updating the data set with the new structures are first-principles calculations carried out. Otherwise, the yet available machine-learned model is used to update the atomic positions. In this manner, most of the first-principles calculations are bypassed during training, and overall, simulations are accelerated by several orders of magnitude while retaining almost first-principles accuracy. In this Perspective, after describing essential components of the active-learning algorithms, we demonstrate the power of the schemes by presenting recent applications.

Hydrogenation Properties of Laves Phases LnMg2 (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb)

The hydrogenation properties of Laves phases LnMg₂ (Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Ho, Er, Tm, Yb) were investigated by thermal analysis, X-ray, synchrotron, and neutron powder diffraction. At 14.0 MPa hydrogen gas pressure and 393 K, PrMg₂ and NdMg₂ take up hydrogen and form the colorless, ternary hydrides PrMg₂H₇ (P4₁2₁2, a = 632.386(6) pm, c = 945.722(11) pm) and NdMg₂H₇ (P4₁2₁2, a = 630.354(9) pm, c = 943.018(16) pm). The crystal structures were refined by the Rietveld method from neutron powder diffraction data on the deuterides (PrMg₂D₇, P4₁2₁2, a = 630.56(2) pm, c = 943.27(3) pm; NdMg₂D₇, P4₁2₁2, a = 628.15(2) pm, c = 940.32(3) pm) and shown to be isotypic to LaMg₂D₇. The LaMg₂D₇ type of hydrides decompose at 695 K (La), 684 K (Ce), 684 K (Pr), 672 K (Nd), and 639 K (Sm) to lanthanide hydrides and magnesium. The Laves phase EuMg₂ forms a hydride EuMg₂H_x of black color. Its crystal structure (P2₁2₁2₁, a = 664.887(4) pm, b = 1136.993(7) pm, c = 1069.887(7) pm) is closely related to the hexagonal Laves phase (MgZn₂ type) of the hydrogen-free parent intermetallic. GdMg₂ and TbMg₂ form hydrides GdMg₂H_x with orthorhombic unit cells (a = 1282.7(4) pm, b = 572.5(2) pm, c = 881.7(2) pm) and TbMg₂H_x (a = 617.8(3) pm, b = 1045.8(8) pm, c = 997.1(5) pm), presumably also with a distorted MgZn₂ type of structure. CeMg₂H₇ and NdMg₂H₇ are paramagnetic with effective magnetic moments of 2.49(1) μ_B and 3.62(1) μ_B, respectively, in good agreement with the calculated magnetic moments of the free trivalent rare-earth cations (μ_calc(Ce³⁺) = 2.54 μ_B; μ_calc(Nd³⁺) = 3.62 μ_B).