Rare earth metal rich magnesium compounds RE4NiMg (RE=Y, Pr–Nd, Sm, Gd–Tm, Lu)—Synthesis, structure, and hydrogenation behavior

The rare earth metal rich compounds RE4NiMg (RE=Y, Pr–Nd, Sm, Gd–Tm, Lu) were synthesized from the elements in sealed tantalum tubes in an induction furnace. All compounds were investigated by X-ray diffraction on powders and single crystals: Gd4RhIn type, space group F4¯3m, Z=16, a=1367.6(2) pm for Y4NiMg, a=1403.7(3) pm for Pr4NiMg, a=1400.7(1) pm for Nd4NiMg, a=1386.5(2) pm for Sm4NiMg, a=1376.1(2) pm for Gd4NiMg, a=1362.1(1) pm for Tb4NiMg, a=1355.1(2) pm for Dy4NiMg, a=1355.2(1) pm for Ho4NiMg, a=1354.3(2) pm for Er4NiMg, a=1342.9(3) pm for Tm4NiMg, and a=1336.7(3) pm for Lu4NiMg. The nickel atoms have trigonal prismatic rare earth coordination. These NiRE6 prisms are condensed via common edges to a three-dimensional network which leaves voids for Mg4 tetrahedra and the RE1 atoms which show only weak coordination to the nickel atoms. The single crystal data indicate two kinds of solid solutions. The RE1 positions reveal small RE1/Mg mixing and some compounds also show Ni/Mg mixing within the Mg4 tetrahedra. Y4NiMg and Gd4NiMg have been tested for hydrogenation. These compounds absorb up to eleven hydrogen atoms per formula unit under a hydrogen pressure of 1 MPa at room temperature. The structure of the metal atoms is maintained with only an increase of the lattice parameters (ΔV/V≈22%) if the absorption is done at T<363 K as at higher temperature a decomposition into REH2–REH3 hydrides occurred. Moreover, the hydrogenation affects drastically the magnetic properties of these intermetallics. For instance, Gd4NiMg exhibits an antiferromagnetic behavior below TN=92 K whereas its hydride Gd4NiMgH11 is paramagnetic down to 1.8 K.