Laves Phases Ln_1–xEu_xMg₂ (Ln = La, Sm) and the Hydride La_0.9Eu_0.1Mg₂H_y≤7

Abstract

Mixed crystal Laves phases Ln1–xEuxMg2 (Ln = La, Sm) were prepared from the elements by melting reactions and characterized by X‐ray powder diffraction. In the substitutional range 0 ≤ x ≤ 0.5 cubic Laves phases of the MgCu2 type, for 0.8 ≤ x ≤ 1 (Ln = La) and 0.9 ≤ x ≤ 1 (Ln = Sm), respectively, hexagonal Laves phases of the MgZn2 type are formed, separated from each other by a miscibility gap. The dependence of the lattice parameters on the degree of substitution x follows closely a Vegard type behavior with a(La1–xEuxMg2 (0 ≤ x ≤ 0.5)) = (8.798(3) + 0.00177(8)x) Å and a(Sm1–xEuxMg2 (0 ≤ x ≤ 0.5)) = (8.629(2) + 0.00404(7)x) Å. Analysis of the unit cell volumes suggests europium to be divalent in all mixed Laves phases. Hydrogenation of the Laves phases as followed by in situ thermal analysis is exothermic for all compositions. For the hexagonal europium rich Laves phases the reaction products upon hydrogenation at 6 MPa hydrogen pressure in an autoclave are binary lanthanum hydride and EuMg2H6, the latter being unsubstituted according to lattice parameters refined by Rietveld analysis. For lanthanum‐rich compounds (0 ≤ x ≤ 0.1) ternary hydrides of the tetragonal LaMg2D7 type structure are formed, if the hydrogen pressure is high enough (11 MPa), and heating rates and reaction temperatures are kept low (373 K). The crystal structure of the new hydride La0.9Eu0.1Mg2Hy≤7 was refined from X‐ray powder diffraction data by Rietveld analysis (space group P41212, a = 6.2592(1) Å, c = 9.3759(3) Å, V = 367.322(2) Å3, Z = 4).

Cd4Cu7As, The First Representative of a Fully Ordered, Orthorhombically Distorted MgCu2Laves Phase

“…Laves phases are of special interest due to their large flexibility in composition and the resulting variety of physical properties. Beside the full range of addressable magnetic properties of binary and pseudo binary rare earth metal containing C15 Laves phases, used for instance in magneto caloric cooling applications, − energy related topics like hydrogen storage are also examined and discussed in the literature. , The well-known basic structure types of Laves phases are the two hexagonal types MgZn 2 (C14) and MgNi 2 (C36) and the cubic MgCu 2 type (C15). − The conditions for the formation of distinct Laves phases can be understood by two general rules, the geometric and the so-called electronic rule . Thus, a widely examined and therefore huge research field is the topology and chemical bonding in Laves phases which is still intensively discussed in literature. − The geometric rule is based on the ideal packing of hard spheres leading to an ideal radii quotient of r A /r B = (3/2) 1/2 = 1.225.…”