Structural properties of two deuterides LaY2Ni9D12.8 and CeY2Ni9D7.7 determined by neutron powder diffraction and X-ray absorption spectroscopy

doi:10.1016/j.jssc.2004.03.034

Journal of Solid State Chemistry

Volume 177, Issue 7, July 2004, Pages 2542-2549

https://doi.org/10.1016/j.jssc.2004.03.034 Get rights and content

Abstract

The crystal structure of two deuterides RY₂Ni₉D_x (R=La; x=12.8 and R=Ce; x=7.7) have been investigated by means of neutron powder diffraction and X-ray absorption spectroscopy. The structures are best described in the space group $R 3 ̄ m$ . The deuterium location has been determined for both compounds. The nature and the occupancy factors of the different D sites are presented. Comparisons are made between the La-based deuteride and the Ce-one in relation with the crystal structure of the intermetallic compounds. Differences in site occupancies within the RM₂ and RM₅ building units of the PuNi₃-type structure are discussed and heterogeneous mixed valence state is reported for the cerium compound.

Introduction

Intermetallic compounds RM_n (R=rare earth or transition metal, M=transition metal, n=1, 2, 3 or 5) are able to store reversibly large amount of hydrogen and are potential materials for energy storage either by solid-gas [1], [2] or electrochemical routes [3], [4]. Nowadays, new interests are devoted to RM₃-type compounds with the recent discovery of new compounds in ternary systems like RM₂Ni₉ (R=rare earth or Ca; M=Mg, Ca, Ti or Y) [5], [6], [7], [8], [9], [10], [11]. These compounds crystallize in an ordered variant of the PuNi₃-type rhombohedral structure ( $R 3 ̄ m$ space group) [12] and can be described as a stacking of CaCu₅ (Haucke phase) and MgZn₂ (Laves phase) units [13] as shown in Fig. 1. Few hydrides have been investigated from a structural point of view in the RM₃ family. For the RNi₃ system (R=Ho, Er, Y), the PuNi₃-type structure is generally preserved upon hydrogenation and neither decomposition nor structural changes are reported. From the neutron diffraction (ND) analysis of HoNi₃D_1.8, Burnasheva et al. [14] found two occupied sites: 71% of the deuterium lies in the interstice 18h₁ and the remaining atoms are located in the site 6c₁. However, for CeNi₃H₃ the hydride phase is formed with a huge uniaxial expansion of the cell (δa/a=−0.80%, δc/c=30.76%) [15]. Recently, Yartys and coworkers [16], [17] found an orthorhombic symmetry lowering into Pmcn space group for the deuteride CeNi₃D_2.7 and reported that deuterium was located only within the CeNi₂ units. In the RCo₃ system, two homogeneity domains are reported for YCo₃. The beta phase YCo₃D_x exists for 1⩽x⩽1.9 and was described in the $R 3 ̄ m$ space group by Benham et al. [18] with significant uniaxial volume expansion (δa/a=−0.07%, δc/c=9.72%) attributed to the filling of the RM₂ units only. Indeed, all deuterium atoms were found in the tetrahedral site 36i₁ in agreement with the exclusive hydrogenation of RM₂ units. The gamma phase YCo₃D_3.8 was investigated by Bartashevich [19]. Less anisotropic volume expansion was observed (δa/a=4.44%, δc/c=8.27%) and, in addition to the site 36i₁ reported for the beta phase, significant occupations for sites 18h₂ and 36i₂ within the RM₅ units were found. A small orthorhombic distortion comparable to that observed in RCo₅ hydrides [20] was also reported. According to these works, it is expected that the beta phase corresponds to the filling of the RM₂ units followed by the filling of the RM₅ units in the gamma phase.

To our knowledge, no structural data have been reported so far on ternary RM₂Ni₉-type deuterides. In this paper, we will present the structural properties of the two deuterides LaY₂Ni₉D_12.8 and CeY₂Ni₉D_7.7 determined by neutron powder diffraction (NPD) and X-ray absorption spectroscopy (XAS). Differences in site occupancies within the RM₂ and RM₅ building units of the PuNi₃-type structure are also discussed.

Section snippets

Experimental

The alloys RY₂Ni₉ (R=La, Ce) were prepared following the procedure described in [10]. Metallographic examination, powder X-ray diffraction experiments (XRD) (Bruker AXS D8 θ–θ diffractometer, CuKα radiation, flat plate, Bragg–Brentano geometry, 2θ-range 20–120°, step size 0.02°, backscattered rear graphite monochromator) and elemental analysis by electron probe micro-analysis (EPMA) were performed to check the homogeneity of the alloys. Deuterated samples have been loaded by solid–gas reaction

Results

The compositions of the alloys obtained by EPMA (see Table 1) are homogeneous and close to the nominal ones. XRD analysis confirms that both samples are single phase and adopt the rhombohedral PuNi₃-type structure as previously described in [10].

This structure can be described as an intergrowth between RM₅ unit (Haücke phase) and RM₂ (Laves phase) ones. Among the two available crystallographic sites for R atoms, lanthanum occupies preferentially the site 3a leading to an ordered ternary

Discussion

The ternary compound LaY₂Ni₉ absorbs more than 1 D/metal, a value very close to the mean one generally observed in other metallic hydrides. Deuterium is equally distributed within the RM_i units and the cell volume expansion is isotropic. The overall δV/V_s (around 26%) are in agreement with value observed in metallic hydrides. If one looks to the nature of the deuterium occupied sites, it is worth to note that they are very similar to those reported for the binary compounds: 4h, 6m, 12n and 12o

Conclusions

The crystal structure of the two deuterides LaY₂Ni₉D_12.8 and CeY₂Ni₉D_7.7 have been determined. It is observed that the lanthanum compound exhibits an isotropic behaviour with occupation of both RM_i units, whereas for the cerium compound anisotropy is related to the exclusive occupation of RM₂ units by deuterium. From these results, it can be established that:

•
Isotropic cell volume expansion is related to full hydrogenation of both RM_i units whereas anisotropic behaviour is obtained when hydrogen

Acknowledgements

The authors wish to thank Mrs. F. Briaucourt and Mr. E. Leroy for technical assistance, Mr. R. Cortes for his help during X-ray absorption experiment in LURE and Mrs. F. Bourée-Vigneron for neutron data acquisition in LLB.

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Structural properties of two deuterides LaY2Ni9D12.8 and CeY2Ni9D7.7 determined by neutron powder diffraction and X-ray absorption spectroscopy

Abstract

Introduction

Section snippets

Experimental

Results

Discussion

Conclusions

Acknowledgements

J. Alloys Compds.

J. Alloys Compds.

J. Alloys Compds.

J. Alloys Compds.

J. Alloys Compds.

J. Solid State Chem.

J. Alloys Compds.

J. Less-Common Met.

J. Less-Common Met.

J. Alloys Compds.

J. Alloys Compds.

Physica B

J. Alloys Compds.

Structural properties of two deuterides LaY₂Ni₉D_12.8 and CeY₂Ni₉D_7.7 determined by neutron powder diffraction and X-ray absorption spectroscopy