Synthesis, crystal structure, and electrochemical hydrogenation of the La2Mg17-xMx (M = Ni, Sn, Sb) solid solutions

Abstract

The crystal structure of La₂Mg_17-xSn_x solid solution was determined by single crystal X-ray diffraction for the first time. This phase crystallizes in hexagonal symmetry with space group P6₃/mmc (a = 10.3911(3), c = 10.2702(3) Å, V = 960.36(6) ų, R₁ = 0.0180, wR₂ = 0.0443 for the composition La_3.65Mg₃₀Sn_1.10) and is related to the structure of CeMg_10.3 and Th₂Ni₁₇-types which are derivative from the CaCu₅-type. A series of isotypical solid solutions La₂Mg_17-xM_x (M = Ni, Sn, Sb, x ~0.8) was synthesized and studied by X-ray powder diffraction, energy dispersive X-ray spectroscopy and fluorescent X-ray spectroscopy. All solid solutions crystallize with the structure related to the Th₂Ni₁₇-type. The electrochemical hydrogenation confirmed the similar electrochemical behavior of all studied alloys. The amount of deintercalated hydrogen depends on the physical and chemical characteristics of doping elements and increases in the sequence Sn < Mg < Sb < Ni. The most geometrically advantageous sites are octahedral voids 6h of the initial structure, thus a coordination polyhedron for H-atom is an octahedron [HLa₂(Mg,M)₄].