Structural and Hydriding Properties of the LaZr2Mn4Ni5-AB3 Type Based Alloy Prepared by Mechanical Alloying from the LaNi5 and ZrMn2 Binary Compounds

In this study, we report on the synthesis of a new AB3-type compound LaZr2Mn4Ni5 (with a content of 40 wt %) at room temperature during 5h of mechanical alloying. This compound was synthesized from the two binary compounds LaNi5 (CaCu5-type structure, P6/mmm space group) and ZrMn2-Laves phase (MgZn2-type structure, P63/mmc space group) in order to take advantage of the hydrogen absorption properties of these two types of intermetallic compounds. Structural properties were investigated using X-ray diffraction (XRD). The surface morphology of the cycled electrode was observed by a scanning electron microscope (SEM). The electrochemical properties of the LaZr2Mn4Ni5–based alloy were determined using the chrono-potentiometry method. The experimental results indicate that the discharge capacity reaches a maximum value of 300 mAh/g. Solid-gaz reaction shows that this compound is able to form the LaZr2Mn4Ni5H13 hydride at room temperature at an absorption plateau pressure of about 7 bar.


Introduction
Intermetallic compounds AB n (A = Y; rare earth, M = transition metal, 1 ≤ n ≤ 5) are able to store reversibly large amount of hydrogen and are therefore potential materials for energy storage. The H 2 absorption-desorption reaction can be performed either by solid-gas or electrochemical routes [1]. Much research have been performed in order to improve the overall properties of the hydrogen storage alloys and to develop new types of hydrogen storage alloys, used as negative electrode materials for the Ni-MH battery. Applications of Ni-MH batteries require continuously an increase in the energy density [2,3]; weight capacities are still low for practical applications and many efforts are conducted worldwide to develop materials with improved performances regarding energy density [4]. It is well known that element substitution is one of the effective methods for improving the overall properties of the hydrogen storage alloys [5], by the replacement of part of the rare earths or the transition metals of the intermetallic compounds by lighter atoms [3].
Kadir [6] have presented a new series of ternary alloys, AMg 2 Ni 9 (where A = La, Ce, Pr, Nd, Sm and Gd), whose structures are built up from alternating MgNi 2 Laves-type phases and rare-earth based AB 5 layers. They found that these compounds crystallize in an ordered variant of the PuNi 3 -type rhombohedral structure (R-3 m space group) [1]. Moreover, it is reported that some of the A-Mg-Ni-based AB 3 -type alloys also exhibited promising electrode properties. For example, the La-Mg-Ni-Co system AB x (x = 3.0-3.5) type quaternary alloys were found to have large discharge capacities of 387-410 mAh/g, higher than those of the commercially used AB 5 -type alloys [7].
In this work we will try to synthesize a new quaternary AB 3type compound LaZr 2 Mn 4 Ni 5 starting from ZrMn 2 Laves phase and LaNi 5 assuming the reaction LaNi 5 + 2 ZrMn 2 → LaZr 2 Mn 4 Ni 5 , La (A atom type) and Ni (B atom type) are then partially substituted respectively by Zr and Mn. According to thermodynamic properties reported for the binary compounds, the intergrowth between LaNi 5 and ZrMn 2 should lead to a compound having intermediate properties between those of the two starting binary compounds. The compound LaNi 5 , which crystallizes in the hexagonal CaCu 5 -type structure (Haucke phase), exhibits exceptional thermodynamical properties toward hydrogen absorption storing up to 6.6 H per formula unit (f.u.) [8]. However, its equilibrium pressure (P = 1.7 bar at room temperature) is too high for practical applications and the molecular mass of La implies a weight capacity limited to 370 mAh/g [1]. In another hand, the use of Zr-based AB 2 alloys as electrodes in nickel-metal hydride batteries has been intensively studied in recent years. This is due to their large hydrogen reversible capacity compared with AB 5 -type compounds presently used in commercial devices [9]. Furthermore, it has been found that the Mn element is beneficial in many respects for the rare earth-based hydrogen storage alloys [10][11][12]. In this work, as it is not possible to obtain the LaZr 2 Mn 4 Ni 5 compound by high-temperature melting synthesis due to the non-miscibility of the two elements La and Zr [3], this compound will be elaborated by mechanical alloying (MA) since it is a process suitable for alloying non-miscible materials and producing powders having a fine microstructural scale [3,[13][14][15][16][17]. In this paper, the structure and the hydrogen storage properties of the mechanically alloyed compound LaZr 2 Mn 4 Ni 5 will be investigated.

Experimental Details
The MA process was carried out using a Fritsch 'Pulverisette 7' planetary ball mill, starting from a mixture of two binary compounds LaNi 5 and ZrMn 2 crushed into powder. These two alloys were prepared by Ultra High Frequency (UHF) induction melting of the pure elements La, Ni and Zr, Mn respectively. The purities of those starting metallic elements were 99.9%, 99.98%, 99.8% and 99.8% respectively. The ingots were melted five times to ensure good homogeneity. For the LaNi 5 compound, the synthesis was carried out under secondary vacuum in a water-cooled copper crucible from a stoichiometric mixture of La and Ni elements with atomic composition 1:5. The synthesis of the ZrMn 2 compound was made under vacuum and then under argon atmosphere when some losses of Mn by sublimation were observed. Binary compounds were analyzed by electron probe microanalysis (EPMA) using a CAMECA SX-100 to ensure their compositions and homogeneity. A mixture of 2 g of these two alloys was then placed in a cylindrical container which was tightly closed under argon atmosphere in a glove box filled with purified argon. The container made by tungsten carbide was loaded with 5 balls (Ø = 12 mm, m = 6.7851 g) with a balls to powder weight ratio equal to 17:1.
The MA conditions correspond to 0.6645.10 -1 J/Hit kinetic shock energy, 93.35 Hz shock frequency and 3.1025 W/g injected shock power [17,18]. The alloy was milled for 5 hours at a disc rotation speed of 450 rpm. The sample obtained was named as S5.
The structural properties of this sample was obtained by X-ray powder diffraction (XRD) that was performed using Cu; Kα radiation at room temperature on a (θ-2θ) Panalytical X'Pert Pro MPD diffractometer. Diffracted intensities were measured in the range 10°-100° with a two-theta step of 0.04° and the diffractogram were fitted using the Rietveld method with the Fullprof software [19][20][21]. The Rietveld method was used to refine the lattice parameters, the atomic parameters (site occupancy and atomic coordinates) and to calculate the weight contents of the existing phases.
Electrochemical properties were tested using a composite negative electrode made of the intermetallic compound as active material, carbon black as electronic conductor and poly-tetra-fluoro-ethylene (PTFE) as a binder. The "latex" technology has been used for the electrode preparation; ninety percent of the alloy powder is mixed with 5% of the black carbon and 5% of the polytetrafluoroethylen (PTFE). Two 0.5 cm 2 pieces of this latex are pressed on each side of a nickel grid current collector to prevent the electrode plate from breaking into pieces during the charge-discharge cycling [22]. This grouping forms the negative electrode of Ni-MH accumulator. The counter electrode was formed by the Ni-oxyhydroxide/dihydroxide (NiOOH/Ni(OH) 2 ), whereas the reference electrode was the Hg/ HgO (1 M KOH). The electrolyte was 1 M KOH solution, prepared with deionised water. All electrochemical measurements were conducted at 25°C in a conventional threeelectrode open-air cell using VMP system. The discharge capacity of the electrode was determined by a galvanostatical charging-discharging for 30 cycles at C/3 and D/6 regime, respectively between -0.6V and -1.3 V versus Hg/HgO. Every cycle was carried out by charging fully at 150 mA/g for 3 h (this time was majored by 50% due to efficiency of the charging reaction) and discharging at 75 mA/g.
The surface morphology of the cycled electrode was observed by a scanning electron microscope (SEM). The pressure-compositiontemperature curve (P-C-T) was measured using a volumetric Sievert's method for pressure between 0.1 and 1 MPa at room temperature [23]. The sample was first activated by 3 hydriding-dehydriding cycles to reduce the grain size and to increase the kinetic (absorption at 25°C under 9 bar and desorption under primary vacuum at 35°C).
In addition, the hydride formation is confirmed by X-ray diffraction measurements carried out after the hydrogen absorption.

Result and Discussions
Structural characterization of the mechanically alloyed compound LaZr 2 Mn 4 Ni 5 The results of the structural characterization of the two starting compounds LaNi 5 and ZrMn 2 shows that LaNi 5 compound is single phase with the composition LaNi 4.98 (3) and crystallizes in the hexagonal CaCu 5 -type structure (P6/mmm space group) with a = 5.0115(1) Å and c = 3.9850(1) Å. For the ZrMn 2 compound, the corresponding XRD pattern can be indexed in the P6 3 /mmc structure with parameters a = 5.0425(1) Å and c = 8.2835(3) Å, more details are given in our previous work [24].
MA starting from LaNi 5 and ZrMn 2 leads to the formation of a nanocrystalline AB 3 -type phase with the hexagonal PuNi 3 -type structure (S.G: R-3m) (Z = 3) [6,25]. This phase is formed in coexistence with ZrMn 2 phase (MgZn 2 -type structure, P6 3 /mmc space group) and a cubic nanocrystalline AB 2 -type phase Zr-Mn-Ni (C15-type) (S.G: Fd-3 m) [26]. Figure 1 shows the XRD pattern refinement of S5 sample for which Rietveld structural parameters are reported in Table 1. As can be seen in Figure 1, the peaks of LaNi 5 compound totally disappear after 5 h of MA whereas those of ZrMn 2 phase are still present. For the formed AB 3 phase with PuNi 3 -type structure, the results show that La atoms are located not only at the 3a site of the PuNi 3 -type structure, but also at the 6c site [24]. Both 3a and 6c sites are then occupied by both La and Zr elements indicating that the alloy is not a fully ordered compound having the same structure as previously reported for AMg 2 Ni 9 compounds [6,25,[27][28][29].
In our previous work, we have showed by transmission electron microscopy (TEM) examinations that the particles size of the mechanically alloyed sample during 5 h is rather inhomogeneous, ranging from 0.5 to 5 µm and the La + Zr/ Mn + Ni ratio have an average about 3 [24]. These results confirm the XRD analysis and show that that the particles produced by MA of LaNi 5 and ZrMn 2 formed a quaternary compound with AB 3 composition. a wide discharge potential plateau based on the oxidation of desorbed hydrogen from the hydride. The mid-discharge potential is about -0.849V. Figure 2b shows the variation of the electrochemical discharge capacity of the AB 3 -based alloy (S5) as function of the number of cycles. However, as this alloy contains additionally to the AB 3 phase, the ZrMn 2 compound and the Zr-Ni-Mn Laves phase (C15-type) with significant mass proportions, it was, therefore, necessary to characterize their reactivity with hydrogen and measure their individual electrochemical discharge capacity to determine their contribution to the total discharge capacity. Figure 3a shows the discharge capacity of the ZrMn 2 compound measured for 30 cycles of charging/discharging by galvanostatic cycling at room temperature and atmospheric pressure to deduce the discharge capacity of the AB 3 -type compound. Therefore ZrMn 2 compound, in these conditions does not contribute to the total discharge capacity determined in Figure 2b for S5 sample based on LaZr 2 Mn 4 Ni 5 compound. According to the literature, the electrochemical discharge capacity of the ZrMn 2 compound is practically zero at 30°C and atmospheric pressure [30]. Nevertheless, it absorbs nearly 4 H/f.u, for maximum pressure of 8 bar, at room temperature, by solid-gas reaction [31]. The electrochemical discharge capacity of the C15 Laves phase Zr-Mn-Ni was also measured (Figure 3b) in order to determine its contribution to the discharge capacity of the S5 alloy. This latter compound was synthesized by UHF induction melting and it's a single phase which structure can be described in the cubic Fd-3 m space group (Figure 4). Figure 3b shows that similarly to the ZrMn 2 compound, the Zr-Mn-Ni Laves phase does not absorb hydrogen and thus does not contribute to the total discharge capacity of the synthesized AB 3 -type based alloy (S5). Thus, the discharge capacity of the prepared alloy depends only of the mass proportion of the LaZr 2 Mn 4 Ni 5 AB 3 -type compound. The corresponding capacity obtained can be therefore expressed in AB 3type phase wheight (mAh/g AB 3 ), its variation versus the cycle number is given in Figure 5.

Electrochemical measurements
According to Figure 5, the LaZr 2 Mn 4 Ni 5 has a good cycle life since the discharge capacities have approximately stable values even after 30 cycles of charging/discharging. The maximum reversible capacity is about 298.56 mAh.g -1 equivalent to 9.12 H/f.u. The discharge capacity of the alloy powder increases to reaches 298.56 mAh/g after 12 cycles and then decreases to 293.6 mAh/g after a few charge-discharge cycles.
The capacity decay of the hydrogen storage alloy electrode is mainly due to the pulverization and oxidation of the active material components to form oxides or hydroxides [32,33]. Figure 6 shows the electrode surface morphologies by SEM of the S5 sample before and after 30 charge-discharge cycles. The alloy consists of large particles and clusters of fine particles. In fact the formed oxide or hydroxide layer acts as a barrier to the hydrogen diffusion and leads to the decrease of the number of hydrogen atoms which can be absorbed by the material. In fact, during the cycling, the rare earth elements such as La or the transition metal such as Mn segregate to the grain boundaries, where they were subject to corrosion. The corrosion products are disposed on the surface of grain particles as La(OH) 3 or as Mn 3 O 4 and act as a barrier to the hydrogen diffusion in the particle volume which in turn decrease the alloy discharge capacity. It was shown that the thickness of the corrosion layer increases generally with the number of charging/discharging cycles [34][35][36].

Gas hydrogenation P-C-T isotherm
The pressure-composition-isotherm (P-C-T) measured at 25°C is presented in Figure 7. The PCT curve of this alloy has two absorption plateau pressures, showing the presence of two types of hydrides A x B y H z and A' x B' y H' z . The first one is characterized by the formation of the first plateau at a pressure of about 7 bar, corresponding to an absorption capacity of 0.6 wt%, the second is characterized by the second plateau at a maximum pressure of about 8 bar corresponding to an absorption capacity of 0.32 wt%. Indeed, according to the literature, ZrMn 2     compound, one of the constituents of the synthesized composite, can absorb nearly 4 H/f.u by solid-gas reaction for maximum pressure of 8 bar, at room temperature [31]. Thus, the second plateau can be attributed to the ZrMn 2 compound and the first one to the LaZr 2 Mn 4 Ni 5 compound. Therefore, by converting the absorption weight capacities to H/f.u capacities, the hydride A x B y H z is LaZr 2 Mn 4 Ni 5 H 13 . We can then conclude that the LaZr 2 Mn 4 Ni 5 compound absorbs hydrogen at room temperature; its absorption capacity in these conditions is in the range of 13 H/uf.
The hydride formation as a result of the solid-gaz absorption of hydrogen by the alloy has been ascertained by XRD analysis. Figure  8 presents the XRD diffractogram of the hydrided alloy. This Figure 8 shows no diffraction peaks of other phases and/or none other of pure metals are observed in the XRD diffractogram of the formed hydride and all the initial phases or compounds before hydrogenation are identified. It is also found that all the hydrided phases still preserve their structures. The hexagonal PuNi 3 -type structure, is preserved for the LaZr 2 Mn 4 Ni 5 compound like in some RMg 2 Ni 9 type alloys [25,27,28].
The lattice parameters and the cell volumes, determined by the the Fullprof program are summarized in Table 2. It is seen that the cell volume has not changed significantly. The reason for this observation is associated with a partial desorption of the solid-gas stored hydrogen in the course of the ex-situ XRD measurement.
This unavoidable desorption, in our case, is a result of hydrogen under pressure driven decomposition initiated upon taking the alloy out of the sample holder, clearly proving that the formed hydride is not stable, it should therefore have an equilibrium plateau pressure greater than atmospheric pressure at room temperature which is explaining the value of 7 bar of the absorption plateau pressure measured for the LaZr 2 Mn 4 Ni 5 compound. Some other AB 3 -type compounds exhibit plateau pressures greater than 1 atm: the LaMg 2 Ni 9 compound has a plateau pressure of H absorbing about 22 bar at 80°C [37], this compound according to Liao [7] shows a plateau pressure of 4 bar at 25°C. Denys [38] have determined the enthalpy value of the hydride formation for this latter compound it is about -22.5 KJ (mol H2 ) -1 at 293 K, this proves that the studied LaMg 2 Ni 9 -AB 3 type compound is also stable with an absorption equilibrium plateau pressure at 122 bar and a desorption plateau pressure at 18 bar at 20°C. These values are in agreement with that of LaZr 2 Mn 4 Ni 5 compound reported in this work.

Conclusion
The hydrogen storage alloy LaZr 2 Mn 4 Ni 5 was successfully synthesized at room temperature, for only 5 h of mechanical alloying, from the mixture of the two binary compounds LaNi 5 and ZrMn 2 at 40 wt.%. The following conclusions can be drawn concerning structural characterization and the hydrogen absorption properties of the mechanically alloyed compound LaZr 2 Mn 4 Ni 5 determined either by solid-gas or electrochemical routes: • XRD analysis show the formation of a composite that is mainly consist of LaZr 2 Mn 4 Ni 5 compound with hexagonal PuNi 3 -type structure coexisting with the ZrMn 2 binary compound (MgZn 2 -type structure C14, P6 3 /mmc space group) and a cubic AB 2 -type Laves phase Zr-Mn-Ni (MgCu 2 -type structure C15, Fd-3 m space group). • The partial substitution of La and Ni by respectively Zr and Mn does not change the main phase structure but affects the lattice parameters. The overall effect is an increase of the cell volume and of c/a ratio as compared to the LaMg 2 Ni 9 compound. • Rietveld analysis proves that LaZr 2 Mn 4 Ni 5 is not fully ordered since both of La and Zr atoms are located simultaneously in the two types of allowed sites 3a and 6c contrarily to the LaMg 2 Ni 9 compound where La atoms occupy only 3a site and Mg atoms the 6c site. • The electrochemical measurements show that the discharge capacity of the synthesized alloy depends only of the mass proportion of the LaZr 2 Mn 4 Ni 5 compound. The corresponding capacity obtained can be therefore expressed in AB 3 -type phase wheight (mAh/g AB 3 ), it is of about 300 mAh/g AB 3 . • The LaZr 2 Mn 4 Ni 5 compound can absorb hydrogen at room temperature; its absorption capacity in these conditions is in the range of 13 H/uf at a plateau pressure of about 7 bar.