SYNTHESIS, STRUCTURE AND THERMAL-STABILITY OF YB4MG4FE3H22

Yb4Mg4Fe3H22 and its deuteride were prepared by sintering mixtures of YbMg, Fe and LiH (LiD) powders in the ratio YbMg:Fe:LiH (LiD) = 4:3:1 at 510-520-degrees-C and 120-155 bar hydrogen (deuterium) pressure, and were characterized by X-ray and neutron powder diffraction. They crystallize with cubic symmetry (space group, P4/mmm; hydride, a = 6.6936(4) angstrom; deuteride, a = 6.6839(5) angstrom, and are isostructural with the calcium analogue Ca4MgFe3H22. The deuteride contains octahedral [FeD6 ]4 - Complex anions with bond distances [Fe-4D1] = 1.586(5) angstrom and [Fe-2D2] = 1.554(7) angstrom, and tetrahedrally coordinated D - anions with bond distances [D3-Mg] = 1.915(9) angstrom and [D3-3Yb] = 2.411(7) angstrom. At a hydrogen pressure of 6 bar, the Yb compound decomposes at about 480-degrees-C into YbH2, Mg and Fe, whereas the Ca analogue decomposes at about 450-degrees-C into Ca2FeH6, Mg and Fe. The enthalpies of desorption, as measured from pressure-composition isotherms, are 137(3) kJ (mol H-2)-1 for Yb4MgFe3H22 and 122(4) kJ (mol H-2)-1 for Ca4MgFe3H22.