Hydrogen induced site depopulation in the LaMgNi4-hydrogen system

The Crystal Structures in Hydrogen Absorption Reactions of REMgNi4-Based Alloys (RE: Rare-Earth Metals)

REMgNi4-based alloys, RE(2−x)MgxNi4 (RE: rare-earth metals; 0 < x < 2), with a AuBe5-type crystal structure, exhibit reversible hydrogen absorption and desorption reactions, which are known as hydrogen storage properties. These reactions involve formation of three hydride phases. The hydride formation pressures and hydrogen storage capacities are related to the radii of the RE(2−x)MgxNi4, which in turn are dependent on the radii and compositional ratios of the RE and Mg atoms. The crystal structures formed during hydrogen absorption reactions are the key to understanding the hydrogen storage properties of RE(2−x)MgxNi4. Therefore, in this review, we provide an overview of the crystal structures in the hydrogen absorption reactions focusing on RE(2−x)MgxNi4.

Crystal Structural Investigations for Understanding the Hydrogen Storage Properties of YMgNi4-Based Alloys

The hydrogen storage properties and crystal structures of YMgNi₄-based alloys, which were synthesized from (2 - x)YNi₂ and xMgNi₂ (0.6 ≤ x ≤ 1.2), were investigated by pressure-composition-temperature measurements and powder neutron diffraction at a deuterium gas pressure to understand the hydrogen absorption and desorption reactions viewed from atomic arrangements around H atoms. Reducing the amounts of MgNi₂, which was utilized as a Mg source in YMgNi₄-based alloys, has been observed to lower the hydrogen absorption and desorption pressures and increase the hydrogen storage capacities. However, the reversible hydrogen capacity attained a maximum value of 1.2 mass % at x = 0.8 because of the formation of a thermodynamically stable hydride in which hydrogen was not released at x = 0.6. In the case of x = 0.6, the presence of excessive Y atoms around the H atoms in the hydrogen-absorbed phase would lead to the formation of a hydride with stronger interaction between Y and H because of the affinity between them. Moreover, the presence of small amounts of D atoms with short interatomic D-D distances (1.6 and 1.9 Å) in the deuterium-absorbed phase (Y_0.81Mg_1.19Ni_4.00D_3.35 and Y_1.06Mg_0.94Ni_4.00D_3.86) at <5 MPa and 323 K was proposed by the crystal structural investigations. The D atoms with short D-D interatomic distances were located in the same local atomic arrangements of D atoms in a deuterium-absorbed phase, which were formed at a higher-pressure range, and had higher hydrogen storage capacities than the deuterium-absorbed phases in this study.

Solid gas and electrochemical hydrogenation properties of the selected R,R’MgNi4-xMx (R,R’ = La, Pr, Nd; M = Fe, Mn; x = 0.5, 1) alloys

New R,R’MgNi4-xMx (R,R’ = La, Pr, Nd; M = Fe, Mn; x = 0.5, 1) phases with cubic structure have been synthesized by powder sintering method. By the hydrogenation of the parent alloys seven hydrides with cubic (LaMgNi3.5Fe0.5H6.0, PrMgNi3.5Mn0.5H~6 and NdMgNi3.5Mn0.5H~5) and orthorhombic (PrMgNi3.5Fe0.5H4.5, NdMgNi3.5Fe0.5H4.3, La0.5Pr0.5MgNi3.5Fe0.5H4.6 and La0.5Nd0.5MgNi3.5Fe0.5H4.4) structures were obtained. The relationship between the structure, hydrogen content and the relative increase of the lattice volume of the new and early known hydrides is shown. Electrochemical parameters of the electrodes based on new materials are compared with already known ones. Highest discharge capacity is observed for PrMgNi3.5Mn0.5 (271 mAh/g) and NdMgNi3.5Mn0.5 (263 mAh/g). The best cyclic stability was seen for the electrode based on NdMgNi3.5Fe0.5 (S50 = 74%).