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

Abstract

The rare earth metal rich compounds RE 4NiMg ( 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: Gd 4RhIn type, space group F4¯3 m, Z=16, a=1367.6(2) pm for Y 4NiMg, a=1403.7(3) pm for Pr 4NiMg, a=1400.7(1) pm for Nd 4NiMg, a=1386.5(2) pm for Sm 4NiMg, a=1376.1(2) pm for Gd 4NiMg, a=1362.1(1) pm for Tb 4NiMg, a=1355.1(2) pm for Dy 4NiMg, a=1355.2(1) pm for Ho 4NiMg, a=1354.3(2) pm for Er 4NiMg, a=1342.9(3) pm for Tm 4NiMg, and a=1336.7(3) pm for Lu 4NiMg. The nickel atoms have trigonal prismatic rare earth coordination. These Ni RE 6 prisms are condensed via common edges to a three-dimensional network which leaves voids for Mg 4 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 Mg 4 tetrahedra. Y 4NiMg and Gd 4NiMg 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 (Δ V/ V≈22%) if the absorption is done at T<363 K as at higher temperature a decomposition into REH 2– REH 3 hydrides occurred. Moreover, the hydrogenation affects drastically the magnetic properties of these intermetallics. For instance, Gd 4NiMg exhibits an antiferromagnetic behavior below T N =92 K whereas its hydride Gd 4NiMgH 11 is paramagnetic down to 1.8 K.