Synthesis, structure and thermal stability of Yb4Mg4Fe3H22

Article abstract of DOI:10.1016/0925-8388(93)90619-X

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 °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) angstroms; deuteride, a = 6.6839(5) angstroms, and are isostructural with the calcium analogue Ca₄Mg₄Fe₃H₂₂. The deuteride contains octahedral [FeD₆]^4- complex anions with bond distances [Fe-4D1] = 1.586(5) angstroms and [Fe-2D2] = 1.554(7) angstroms, and tetrahedrally coordinated D^- anions with bond distances [D3-Mg] = 1.915(9) angstroms and [D3-3Yb] = 2.411(7) angstroms. At a hydrogen pressure of 6 bar, the Yb compound decomposes at about 480 °C into YbH₂, Mg and Fe, whereas the Ca analogue decomposes at about 450 °C into Ca₂FeH₆, Mg and Fe. The enthalpies of desorption, as measured from pressure-composition isotherms, are 137(3) kJ (mol H₂)^-1 for Yb₄Mg₄Fe₃H₂₂ and 122(4) kJ (mol H₂)^-1 for Ca₄Mg₄Fe₃H₂₂.