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Solid gas and electrochemical hydrogenation properties of the selected

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%).


Introduction
Intermetallic RMgM4 phases (R -rare earth metal, M -transition d-metal) and their alloys have been intensively studied during the last ten years [1]. They are paid attention not only of their structural features and physical properties, but also of their hydrogen sorbtion properties [2]. Most of the RMgM4 alloys reversibly absorb hydrogen from the gas phase and electrochemically. The amount of absorbed hydrogen can reach up to 6 H/f.u. Highest discharge capacity of the electrodes for these materials can reach 400 mAh/g [3]. Although there are many studies on the hydrogen adsorption properties of the multi phase alloys which containg RMgM4 intermetallics [4], a comprehensive study of single-phase RMgM4 samples is limited to a few dozen works, most of which are devoted to the study of stoichiometric 1:1:4 composition [5][6][7][8][9][10][11][12][13]. In our recent studies we shown the effect of the rare earth and transition metals substitution on the hydrogen sorbtion properties based on the LaMgNi4 compound [14][15]. We also turn our attention to the investigation alloys containg heavy rare earth metals and different multiple substitution of the transition metals [16][17]. In present work, we synthesized RMgNi4 alloys (R = La, Pr, Nd), where the nickel atoms is partially replaced by Mn and Fe ones. The influence of such substitution on hydrogen sorption properties and hydride structures is shown, and the obtained results are compared and briefly disscused.

Experimental part
Starting materials for the preparation of the R,R'MgNi4-xMx alloys were ingots of La, Pr, Nd, Ni, Fe and Co (all with purity ≥99.9%) and Mg powder (325 mesh, 99.8%). In the first step, R,R'Ni4-xMx (x = 0.5 and 1) precursors were prepared by arc melting in a purified argon atmosphere. The as-cast alloys were ground in an agate mortar and after obtained powders with size less than 0.04 mm then mixed with Mg powder in certain proportions.
Mg was added with 3 wt. % excess to compensate for its evaporation loss at high temperatures. The powder mixtures were pressed into pellets with diameter of 10 mm at 10 ton, placed into stainless steel containers and sealed under an Ar atmosphere. Samples further heated up to 800ºC and down to 500 ºC step by step within six days, and annealed at 500ºC ten days. No reaction with the containers was observed. The composition of the samples was controlled by comparing the mass of the obtained alloys with the total mass of the initial load (~2 g). The mass losses during sintering did not exceed 1 %. Structural analysis of the samples was carried out by X-ray powder diffraction (XRD) using a DRON 3M diffractometer (Cu Kα radiation). The scans were taken in the /2 mode with the following parameters: 2 region 20-90•, step scan 0.04°, counting time per step 10 s. Crystal structures of the compounds were refined by Rietveld method from the diffraction data using Fullprof software [18]. A pseudo-Voigt profile shape function was assumed. The background was refined with a polynomial function. LaMgNi3

Results and discussion
Sample synthesis and crystal structure of parent alloys By using powder sintering method a series of the dopped RMgNi4 alloys were prepared. Single phase was observed in the Febased RMgNi3.5Fe0.5 (R = La, Pr, Nd) and La0.5R0.5MgNi3.5Fe0.5 (R = Pr, Nd) and Mn-based RMgNi3.5Mn0.5 and RMgNi3Mn (R = La, Pr, Nd) alloys. These phases belong to the cubic SnMgCu4 structure type (space group F-43m) and their lattice parameters are given in Table 1.
In the SnMgCu4 structure, the R atoms occupied     structures. Synthesized hydrides with orthorhombic structure were obtained in crystalline form, while the cubic ones exibit the crystallinity of low level (see Figure 1 (b)). XRD of the hydrides were also analyzied to determine the dimension of the crystallites and the lattice distorsion. Size (L) and strain (e) parameters (Table 3) were obtained from the equation [20]: Up to our best knowledge, the structure only for one reprecentative, the LaMgNi4H0.75 hydride, has been studied.

Electrochemical properties
Electrochemical studies of all synthesized alloys were performed in the cycling manner at charge and discharge current densities of 50 mA/h. Discharge capacities as a function of discharge cycle are illustrated in Figure 5. Some electrochemical properties are listed in Table 4.

Conclusions
The influence of partial substitutions of