Hydrogen induced site depopulation in the LaMgNi₄-hydrogen system

Abstract

Metal hydride structures / In-situ neutron powder diffraction / Pressure-composition-isotherms / Structure-property relation Abstract. The LaMgNi 4 -hydrogen system was investigated by in-situ neutron powder diffraction and pressurecomposition isotherm measurements at 100 C and hydrogen (deuterium) pressures of up 50 bar. The system displays three hydride phases that have distinctly different hydrogen plateau pressures and H atom distributions. The cubic a-LaMgNi 4 H 0:75 phase forms below 0.01 bar hydrogen pressure and H atoms fill one type of tetrahedral Ni 4 interstices. The orthorhombic distorted b-LaMgNiH 3:7 phase forms at $3 bar hydrogen pressure and H atoms fill both tetrahedral LaNi 3 and triangular bi-pyramidal La 2 MgNi 2 interstices. Interestingly, tetrahedral Ni 4 interstices are no longer occupied. Finally, the most hydrogen rich gLaMgNi 4 H 4:85 phase forms above 20 bar. It has again cubic symmetry and H atoms continue to occupy triangular bipyramidal La 2 MgNi 2 interstices while filling a new type of tetrahedral Ni 4 interstices that are neither occupied in the a-nor in the b-phase. The tetrahedral LaNi 3 interstices occupied in the b-phase are empty. Hydrogen induced depopulations of interstitial sites in metal hydrides are relatively rare and consistent with, but not entirely due to, the onset of repulsive H--H interactions at increasing hydrogen concentrations.

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

“… 4 − 11 In the case of RE (2 – x ) Mg x Ni 4 , the H atom is in the elemental state, which is qualitatively associated with less interaction between the metal elements and H than MgH 2 . Then, the subsequent hydrogen absorption reaction steps of LaMgNi 4 at 373 K in a hydrogen (deuterium) atmosphere have been reported 6 …”

“…Figure 1 shows the crystal structures of LaMgNi 4 obtained during the hydrogen absorption reaction. 6 , 10 …”

Effect of magnesium on the crystal structure and thermodynamics of the La3−xMgxNi9 hydrides

“…This fact implies a possible formation of the local Mg-D bonding in the La 3−x Mg x Ni 9 (H,D) 9-13.5 deuterides. The other interesting example of the formation of the MgD 6 coordination is the structure of the ␥-deuteride of the LaMgNi 4 compound (MgCu 4 Sn type, ordered derivative of the Laves MgCu 2 type) studied by Chotard et al [27]. In the cubic ␥-LaMgNi 4 D 4.58 deuteride it was observed nearly a complete filling of the two types of the D sites: D 1 occupies a bipyramid [La 2 MgNi 2 ] and D 2 is in the tetrahedral [Ni 4 ] interstice [27].…”

“…The other interesting example of the formation of the MgD 6 coordination is the structure of the ␥-deuteride of the LaMgNi 4 compound (MgCu 4 Sn type, ordered derivative of the Laves MgCu 2 type) studied by Chotard et al [27]. In the cubic ␥-LaMgNi 4 D 4.58 deuteride it was observed nearly a complete filling of the two types of the D sites: D 1 occupies a bipyramid [La 2 MgNi 2 ] and D 2 is in the tetrahedral [Ni 4 ] interstice [27]. Analysis of the structure of ␥-LaMgNi 4 D 4.58 shows that D atoms form a regular octahedron around the Mg atoms (with Mg-D distances of 2.01Å) and a tetrahedron around the Ni atoms.…”

Solid gas and electrochemical hydrogenation properties of the R1-xR’xMgNi4-yCoy (R, R’ = Y, La, Ce) alloys

“…Basically obtained materials are the mixtures of Laves AB2, Haucke AB5 or/and superlattice (AB2)n(AB5)m phases. Among them the RMgNi4 phases are characterized by good solid gas and electrochemical properties and can be easily prepared by sintering or induction melting methods [6][7][8][9][10][11][12][13][14][15][16][17][18]. Recently, the YMgNi4-xCox [19], LaMgNi4-xCox [20], NdMgNi4-xCox [21], La1-xNdxMgNi4-yCoy [22] and Pr1-xLaxMgNi4-yCoy [23] alloy systems were systematically investigated in our research group.…”