Crystallographic and hydriding properties of the system La1−xCexY2Ni9 (xCe=0, 0.5 and 1)

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Abstract

The structural properties of the system La1−xCexY2Ni9 with xCe=0, 0.5 and 1 have been investigated by electron probe microanalysis, powder X-ray diffraction and absorption spectroscopy. The compound LaY2Ni9 adopts a rhombohedral structure of PuNi3-type (R-3m space group, Z=3). It can be described as an intergrowth between RM5 (Haücke phase) and RM2 (Laves phase) type structures. Among the two available crystallographic sites for R atoms, lanthanum occupies preferentially the site 3a leading to a partially ordered ternary compound. Substitution by cerium involves anisotropic variations of the cell parameter with a decrease of a and an increase of c leading to an overall cell volume reduction. Increasing cerium content does not induce any symmetry change but leads to a statistical distribution of the rare earths over the two sites 3a and 6c involving an evolution toward a pseudo-binary compound. This behavior is related to the intermediate valence state of cerium observed by X-ray absorption spectroscopy. The hydriding properties of the two compounds LaY2Ni9 and CeY2Ni9 are described in relation with their crystallographic structure.

Introduction

Intermetallic compounds RMn (R=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 H2 absorption–desorption reaction can be performed either by solid-gas or electrochemical routes. It has allowed the development of competitive negative electrode materials to prepare NiMH-type batteries [1] that now widely replace the NiCad ones in the huge market of portable goods. Most of the researches have been focused in the past on binary RM5-, RM2- and RM-type alloys. However, these systems still suffer from insufficient capacities, passivation, slow activation and corrosion [2].

To our knowledge, few works have been devoted to ternary compounds. Recently, Kadir et al. [3], [4], [5], [6] reported on the R–Mg–Ni system (R=La, rare earth, Ca or Y) with atomic composition 1:2:9. They found that these compounds crystallize in an ordered variant of the PuNi3-type rhombohedral structure (R-3m space group). Among the two available sites 3a and 6c for elements R, a strong preference for site 3a is observed whereas Mg is sited on site 6c. That leads to the conclusion that these compounds should be considered as fully ordered ternary compounds (RMg2Ni9) rather than pseudo-binary RM3-type ones (R0.33Mg0.66Ni3) since the repartition of R and Mg is not random. In a matter of fact, the structure can be described as a stacking of RNi5 (Haucke phase) and MgNi2 (Laves phase) units following the scheme: RNi5+2 MgNi2RMg2Ni9 [7].

Following this scheme, the La–Y–Ni system has been investigated in the present work assuming the reaction: LaNi5+2YNi2 → LaY2Ni9. Both LaNi5 and YNi2 are well known to react readily with gaseous hydrogen. The compound LaNi5, which crystallizes in the hexagonal CaCu5-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. The structural properties of YNi2 have been studied in detail in the past [9], [10]. It was shown that this Laves phase is in fact a non-stoichiometric compound with exact formulation Y0.95Ni2. The non-stoichiometry was explained by ordered vacancies distributed on the Y sublattice and involving a symmetry reduction in space group F-43m. Van Essen and Buschow [11] have investigated the thermodynamical properties of the hydride. They found a solid gas capacity of 3.6 H/f.u. No plateau pressure was observed but the pressure-composition curve was found to increase almost linearly in the range 10−4 to 10 bar at 50°C indicating that the hydrogen content depends almost linearly on the pressure. More recently, the crystal structure of the deuteride Y0.95Ni2D2.6 was studied by neutron diffraction [12]. From these authors, crystalline hydride can be obtained up to 2.6 D/f.u. at 1 bar and 25°C but any attempt to prepare a deuteride with higher deuterium content leads to a partial amorphization of the compound. Such behavior is commonly observed for R1−xNi2 hydrides when increasing hydrogen content [13].

According to the thermodynamic properties reported for the binary compounds, the intergrowth between LaNi5 and YNi2 should lead to a compound having an equilibrium pressure close to atmospheric pressure at room temperature and a capacity of about 12 H/f.u. Following this scheme, the compound LaY2Ni9 has been prepared and studied in this work. Mischmetal is often used in industrial alloys for economical purpose since this mixture of light rare earths is cheaper than pure lanthanum. However it appears that mischmetal composition (and particularly the amount of cerium) is also of great importance for hydride properties [14]. For that reason, the possible replacement of lanthanum by cerium has also been studied and the influence of Ce substitution will be described from a structural and thermodynamical point of view.

Section snippets

Experimental section

The samples were prepared by induction melting of the pure components (3N purity) under vacuum in a water-cooled copper crucible. The samples were melted five times to ensure good homogeneity. They were then annealed at 750°C for 3 weeks and quenched to room temperature.

Metallographic examination and elemental analysis by electron probe micro-analysis (EPMA Camebax SX100) were performed to check the homogeneity of the alloys. The powder X-ray diffraction experiments (XRD) were performed at room

The compound LaY2Ni9

First attempt to prepare the compound LaY2Ni9 was made using an annealing treatment at 900°C. From microprobe analysis, it was found that the main phase was indeed LaY1.8Ni8.7 in fairly good agreement with the nominal composition. However, metallographic examination, EPMA and X-ray diffraction indicate the presence of secondary phases beside the main phase. Those phases were identified to be of R2Ni7 and RxNi2-type, in fact the neighbor phases of RNi3 if one refers to the La–Ni phase diagram.

Effect of cerium substitution

From metallographic observation, it is found that the compounds with xCe=0.5 and 1 are single phase. The results of the EPMA analysis for all compounds are given in Table 1. It confirms the stoichiometry 1:2:9 for the main phase.

These results are confirmed by XRD analysis. For all samples, the main phase can be indexed in the rhombohedral cell of PuNi3-type. The cell parameters for the La1−xCexY2Ni9 system are given in Table 2 and their evolutions are shown in Fig. 2.

The total replacement of

Discussion

From our structural study, it can be concluded that the intergrowth between LaNi5 and YNi2 to form a new compound of formulation LaY2Ni9 is possible. From EPMA (Table 1), the stoichiometry for yttrium is equal to 1.98(1). As stated above, the compound YxNi2 is under-stoichiometric and exists only for x=0.95. As the RY2Ni9 structure is obtained from a stacking of CaCu5 and MgZn2 units and because the crystallographic study has shown that a large part of the yttrium atoms are sited in the RM2

Conclusions

To conclude, the structural properties of the ternary system La1–xCexY2Ni9 with x=0; 0.5 and 1 have been investigated. The compounds adopt a rhombohedral structure of PuNi3-type, which can be described as an intergrowth between RM5 and RM2-type structures. It is shown that depending of the atomic radius difference between yttrium and the pseudo-atom R=(La1−xCex) either a partially ordered ternary or a randomly distributed pseudo-binary compound can be obtained. From this assumption, the

Acknowledgements

We thank Mrs. F. Briaucourt and Mr. E. Leroy for technical assistance and Mr. R. Cortes for his help during X-ray absorption experiment.

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