Crystal chemistry and metal-hydrogen bonding in ...folk.uio.no/ravi/cutn/totpub/47.pdf · Crystal chemistry and metal-hydrogen bonding in anisotropic and interstitial hydrides of

Crystal chemistry and metal-hydrogen bonding in anisotropicand interstitial hydrides of intermetallics of rare earth (R)and transition metals (T), RT3 and R2T7

1

Volodymyr A. Yartys*, I, Ponniah VajeestonII, Alexander B. RiabovI, Ponniah RavindranII, Roman V. DenysI,Jan Petter MaehlenI, Robert G. DelaplaneI and Helmer FjellvagII

I Institute for Energy Technology, P.O. Box 40, 2027 Kjeller, NorwayII Department of Chemistry, P.O. Box 1033, University of Oslo, Blindern, 0315 Oslo, Norway

Received May 16, 2008; accepted November 7, 2008

Metal hydrides / Crystal structures / Bonding mechanism /Nickel / Rare earth metals

Abstract. Hydrides of the RNi3- and R2Ni7-based (R¼light rare earth element) intermetallics exhibit novel struc-tural features. Structures of these hydrides, includingCeNi3D2.8, La2Ni7D6.5, LaNi3D2.8, and Ce2Ni7D4.7, areformed via a huge volume expansion occurring along asingle crystallographic direction. Unique structural featuresduring the formation of the hydrides include: (a) The lat-tice expansion proceeds exclusively within the RNi2 slabsleaving the RNi5 slabs unmodified. Such expansion,�60% along [001] for the Laves layers, is associated withoccupation of these slabs by D atoms; (b) New types ofinterstitial sites occupied by D are formed; (c) An orderedhydrogen sublattice is observed. In the present work wegive (a) a review of the crystal chemistry of the conven-tional, interstitial type hydrides formed by RT3 and R2T7

intermetallic compounds (R ¼ rare earths; T ¼ Fe, Co, Ni)as compared to the unusual features of the crystal chemis-try of anisotropic hydrides formed by the RNi3 and R2Ni7intermetallics and (b) studies of the interrelation betweenstructure and bonding in anisotropic hydrides by perform-ing density functional calculations for CeNi3 and Ce2Ni7intermetallic alloys and their corresponding hydrides.These studies provide an understanding of the bondingmechanism in the hydrogenated compounds which causesa complete anisotropic rebuilding of their structures. FromDOS analysis, both initial intermetallics and their relatedhydrides were found to be metallic. Bader topological ana-lysis for the non-hydrogenated intermetallics showed thatCe atoms donate in average of almost 1.2 electrons to theNi sites. Hydrogenation increases electron transfer fromCe; its atoms donate 1.2–1.6 electrons to Ni and H. TheCharge Density Distribution and Electron LocalizationFunction for the Ce2Ni7D4.7 phase clearly confirm that theinteraction between the Ce and Ni does not have any sig-

nificant covalent bonding. Ni is bonded with H via form-ing spatial frameworks ––H––Ni––H––Ni–– where H atomsaccumulate an excess electron density of �0.5e�. Thus,the tetrahedral or open saddle-type NiH4 coordination ob-served in the structures of these hydrides is not associatedwith the formation of [Ni0H4

1�]4� complexes containing ahydrido-ion H�1. In the structural frameworks there areterminal bonds Ni––H, bridges Ni––H––Ni, and the bondswhere one H is bound to three different Ni. These spatialordered frameworks stand as the principal reason for theanisotropic changes in the structural parameters on hydro-genation. Another unique feature of anisotropic hydrides isthe donation of electrons from nonhydrogenated RNi5 partsto hydrogen in RNi2 slabs stabilising these fragments.

Introduction

Hydrogen absorption by intermetallic compounds (IMC)results in a storage of atomic hydrogen in the metal lat-tice frequently reaching a high ratio of H/M (>1) and ahigh volume density of the stored hydrogen compared tocompressed or liquefied hydrogen gas. Intermetallic hy-drides exhibit a close interrelation between crystal chem-istry and hydrogen sorption behaviour allowing alterationand optimisation of their H storage performance. Hydro-gen accommodation by the metal lattice is typically ac-companied by modest (few percent) changes of the in-teratomic metal–metal distances. H atoms enter theinterstices, which are originally available in the virginintermetallics. However, this “typical” case does not cov-er a large group of very interesting and so far insuffi-ciently studied compounds, the so-called “anisotropic”hydrides. In such hydrides, a huge expansion proceeds ina specific crystallographic direction upon hydrogenationand leads to a dramatic differentiation of the propertiesof the hydrides along the direction of the expansion andnormal to it. This paper will review recent data on the“anisotropic” hydrides of intermetallic compounds incomparison with conventional interstitial intermetallic hy-drides formed by chemically related intermetallic com-

674 Z. Kristallogr. 223 (2008) 674–689 / DOI 10.1524/zkri.2008.1030

# by Oldenbourg Wissenschaftsverlag, Munchen

1 Presented at the International Symposium on Metal-HydrogenSystems MH2008. Fundamentals and Applications. Reykjavik, Ice-land. June 24–28, 2008. Lecture F4-O-4.

* Correspondence author (e-mail: [email protected])

pounds of rare earth elements with Ni/Co/Fe. Results ofelectronic structure calculations will be presented for ani-sotropic hydrides, formed by CeNi3 and Ce2Ni7 interme-tallic compounds and discussed as related to their unu-sual structural features.

The intermetallics in the systems of rare earth metals(R) with Ni, Co or Fe (T), are frequently formed betweencompositions RT2 (Laves compounds) and RT5 (Hauckephases). Their composition RTa (2 < a < 5) can be achievedfrom a combination of RT5 (� n) and R2T4 (�m) units.These include, for example, R2T4 + RT5 ¼ 3 � RT3 andR2T4 + 2 � RT5 ¼ 2 � R2T7 [1, 2]. The intermediate com-pounds crystallise with several types of structures, whichare built from the slabs of Laves and Haucke types stack-ing along the hexagonal/trigonal c-axis. Consequently,their structures are considered as hybrid ones. Particularfocus of this paper will be on a review of the data obtainedfor R(Ni,Co,Fe)3 and R2(Ni,Co)7 compounds. These hybridstructure types include PuNi3, CeNi3, Ce2Ni7 and Gd2Co7

[1]. As example, crystal structures of the CeNi3, PuNi3 andCe2Ni7 types are shown in Fig. 1 as an alteration along[001] of the RT5 CaCu5-type (coloured) and RT2 Laves-type slabs. Hydrogen interaction with these hybrid interme-tallic structures has been studied rather extensively (seeTable 1). These studies include also neutron powder dif-

fraction investigations, thus allowing to determine the struc-ture of the hydrogen sublattice in the deuterated materials.The known crystal structures of isotropic and anisotropichydrides of such compounds are presented in Table 2.

Hydrogenation of intermetallic compounds RT3 andR2T7 proceeds via two or three different mechanisms. Atwo-step hydrogen uptake occurs for the RNi3, RCo3 andR2Co7 intermetallics leading to the formation of lower hy-drides (dihydrides RNi3H1.2-2.0, RCo3H1.3–2.1 and trihy-drides R2Co7H1.5–2.7) prior to the formation of saturatedtetrahydrides RNi3H3.4–4.3, RCo3H3.6–4.6 and hexahydridesR2Co7H5.8–6.6. Continuous increase of the H content in theRFe3-based hydrides proceeds within a single phase areaand gives hydrides with H/RFe3 changing from 1.5 to 4.8.An alternative mechanism of the formation of saturatedhydrides is observed for the compounds of La and Ce,where a single-step hydrogen absorption process leads tothe formation of R(Ni,Co)3H2.7–6.0 and R2(Ni,Co)7H4.1–6.5.

From a crystallographic point of view, these schemeshave distinct differences in the way the intermetallic struc-tures are transformed into the hydrides and can be classi-fied in the following three groups:

Type I. Lower hydrides RNi3H1.2–2.0, RCo3H1.3–2.1 andR2Co7H1.5–2.7. Moderate expansion of the original latticesproceeds anisotropically, along [001]. Linear expansion of6.5–11.5% corresponds to the volume expansion of 6.9–12.2%. DV/at. H ¼ 2.1–4.6 �A3.

Type II. Higher hydrides RNi3H3.4–4.3, RCo3H3.6–4.6

and R2Co7H5.8–6.6 are formed by H uptake by the lowerhydrides RNi3H1.2–2.0, RCo3H1.3–2.1 and R2Co7H1.5–2.7.Nearly similar lattice expansion proceeds along [001] andin the basal plane. Thus, lattice expansion is ratherisotropic and gives volume increase of 11.8–26.6%. DV/at. H ¼ 2.3–4.1 �A3.

For the isostructural RFe3 intermetallics, higher valuesof the maximum hydrogen storage capacity are achieved,up to 4.8 at. H/f.u. in the case of YFe3. The tetrahydridesRFe3H�4 are similar to the RCo(Ni)3H�4 hydrides in theircrystallographic characteristics; lattice expansion proceedsboth in basal plane and along [001]. Hydrogenation leadsto a more pronounced enlargement of c compared to a;thus, c/a increases to 4.83–5.02 from the original 4.73–4.90 for the intermetallic compounds. If hydrogenationtemperatures exceed a certain critical temperature, continu-ous phase transformations from hydride with H content of1.5–1.8 at. H in DyFe3 and ErFe3-based hydrides to thesaturated values of 4.0–4.2 take place. In such a case achange in hydrogen content in RFe3Hx (M ¼ Dy, Er) [20,21] is accompanied by a continuous increase in the unitcell dimensions. This does not lead to significant changesin c/a or specific volume of H, DV/at. H, which remainsin the window 2.3–3.8 �A3/at. H. This range of values islower than for RNi3H3.4-4.2 and RCo3H3.6-4.0 tetrahydrides(3.0–4.1 �A3/at. H), as the unit cell volumes of the initialintermetallics are much larger for RFe3 compared to thecorresponding compounds Ni or Co.

Type III. Hydrides of La and Ce compounds,R(Ni,Co)3H2.7–5.2 and R2(Ni,Co)7H4.1–6.5, are formed byanomalously high linear expansion along [001] reaching35.8%. Since the basal plane remains practically un-changed, this gives similar values for the volume expan-

Crystal chemistry and metal-hydrogen bonding 675

Fig. 1. Stacking of the RT2 Laves-type and RT5 Haucke-type layers(coloured) in the hybrid crystal structures of CeNi3, LaNi3 andCe2Ni7 types (ac projection). The structures contain nets if threetypes stacking along [001] including RT2 (plain, inside the RT5 type),R2T (buckled, inside the RT2 type) and T3 (plain, on the borders be-tween RT5 and RT2 and RT5 and RT5).

676 V. A. Yartys, P. Vajeeston, A. B. Riabov et al.

Table 1. Crystallographic characteristics of RT3 and R2T7 compounds and their hydrides.

IMC Type* Alloy Hydride Da/a, % Dc/c, % DV/V, % DV/at. H, �A3 Ref.a c c/a H/f.u. a c c/a

RNi3 with PuNi3 structure type

CaNi3 II 5.030 24.27 4.83 4.6 5.444 26.56 4.88 8.2 9.4 28.2 3.6 [3]

YNi3 I 4.977 24.44 4.91 1.6 4.987 26.82 5.38 0.2 9.7 13.2 4.6 [4]

YNi3 II 4.973 24.37 4.90 4.0 5.267 26.57 5.05 5.9 9.0 22.3 3.2 [5]

LaNi3 III 5.082 25.09 4.94 2.8 8.6408b¼ 4.9281b¼ 90.85�

32.774 6.596.65

––1.8––3.0

30.6 24.3 5.4 [6]

GdNi3 II 5.009 24.57 4.91 2.0 5.182 24.81 4.79 3.5 1.0 8.1 2.4 [7]

GdNi3 III 5.009 24.57 4.91 3.0 4.914 31.41 6.39 ––1.9 27.8 23.0 4.5 [4]

TbNi3 II 4.979 24.48 4.92 4.2 5.302 26.78 5.05 6.5 9.4 24.0 3.3 [7]

DyNi3 I 4.980 24.44 4.91 2.2 5.039 26.67 5.29 1.2 9.1 11.9 3.1 [4]

II 3.4 5.280 26.74 5.07 6.0 9.4 23.0 3.9 [4]

HoNi3 I 4.953 24.21 4.89 1.8 4.991 26.12 5.23 0.8 7.9 9.6 3.0 [8]

II 3.6 5.305 26.70 5.03 6.9 10.0 20.4 4.1 [4]

HoNi3 I 4.954 24.33 4.91 1.3 4.986 26.08 5.23 0.63 7.2 8.6 3.8 [9]

I 1.8 5.027 26.24 5.22 1.45 7.9 11.0 3.5 [9]

II 3.7 5.258 26.71 5.08 6.1 9.8 23.6 3.7 [9]

ErNi3 I 4.951 24.27 4.90 1.9 4.980 25.85 5.19 0.6 6.5 7.8 2.3 [7]

ErNi3 I 4.948 24.29 4.91 1.23 4.972 25.90 5.21 0.5 6.6 7.7 3.6 [10]

I 1.97 5.046 26.16 5.18 2.0 7.7 12.0 3.5 [10]

II 3.75 5.240 26.61 5.08 5.9 9.5 22.8 3.5 [10]

ErNi3 II 4.943 24.28 4.91 4.0 5.271 26.65 5.06 6.6 9.8 24.8 3.5 [11]

II 5.0 5.294 26.7 5.04 7.1 10.0 26.1 3.0 [11]

TmNi3 I 4.935 24.26 4.92 2.0 4.981 25.85 5.19 0.9 6.6 8.6 2.4 [7]

CeY2Ni9D7.7 III 4.971 24.54 4.94 2.57 4.872 31.312 6.43 ––2.0 27.6 22.6 5.1 [12]

LaY2Ni9D12.8 II 5.034 24.51 4.87 4.27 5.396 26.885 4.98 7.2 9.7 26.0 3.6 [12]

RCo3 with PuNi3 structure type

YCo3 I 5.013 24.35 4.86 2.0 5.000 26.23 5.25 ––0.3 7.7 7.2 2.1 [13]

II 3.7 5.268 26.46 5.02 5.1 8.7 20.0 3.2 [13]

YCo3 I 5.013 24.35 4.86 1.0 5.013 25.86 5.16 0.0 6.2 6.2 3.7 [14]

YCo3 II 5.018 24.38 4.86 3.8 5.241 26.401 5.04 4.4 8.3 18.1 2.8 [15]

YCo3 I 5.015 24.38 4.86 1.3 5.0209 25.9569 5.17 0.1 6.5 6.7 3.0 [16]

I 2.0 4.9992 26.9295 5.39 ––0.3 10.5 9.8 2.9 [16]

II 4.6 5.2666 26.7753 5.08 5.0 9.8 21.1 2.7 [16]

CeCo3 III 4.955 24.75 5.00 4.0 4.956 32.69 6.60 0.0 32.1 32.1 4.7 [14]

CeCo3 III 4.960 24.80 5.00 4.0 4.936 32.45 6.57 ––0.5 30.8 29.5 4.3 [11]

III 6.0 4.98 32.65 6.56 0.4 31.7 32.7 3.2 [11]

PrCo3 I 5.068 24.79 4.89 1.8 5.091 27.57 5.42 0.5 11.2 12.2 4.2 [13]

II 3.7 5.380 27.30 5.07 6.2 10.1 24.1 3.9 [13]

NdCo3 I 5.055 24.70 4.89 1.8 5.055 27.40 5.42 0.0 10.9 10.9 3.7 [13]

II 4.0 5.357 27.24 5.09 6.0 10.3 23.9 3.6 [13]

GdCo3 I 5.037 24.51 4.87 2.0 5.021 27.34 5.45 ––0.3 11.5 10.8 3.2 [13]

II 3.7 5.291 26.88 5.08 5.0 9.7 21.0 3.4 [13]

TbCo3 I 5.016 24.43 4.87 2.0 5.005 26.80 5.36 ––0.2 9.7 9.2 2.7 [13]

II 3.8 5.262 26.85 5.48 4.9 18.1 20.9 3.3 [13]

DyCo3 I 5.007 24.27 4.85 2.1 4.992 26.61 5.33 ––0.3 9.6 9.0 2.5 [13]

II 3.7 5.233 26.66 5.10 4.5 9.8 20.0 3.2 [13]

HoCo3 I 4.985 24.22 4.86 2.0 4.990 26.12 5.23 0.1 7.8 8.1 2.3 [13]

HoCo3 II 3.8 5.233 26.33 5.03 5.0 8.7 19.8 3.0 [13]

ErCo3 I 4.977 24.26 4.87 1.8 4.981 25.83 5.19 0.1 6.5 6.6 2.1 [13]

II 3.6 5.221 26.27 5.03 4.9 8.3 19.3 3.1 [13]

II 4.1 5.217 26.123 5.01 4.8 7.7 18.3 2.6 [13]

ErCo3 I 4.980 24.25 4.87 1.37 4.987 26.057 5.22 0.2 7.4 7.8 3.3 [17]

II 3.71 5.222 26.055 4.99 4.9 7.4 18.1 2.8 [17]


Table 1. Continued.

IMC Typea Alloy Hydride Da/a, % Dc/c, % DV/V, % DV/at. H, �A3 Ref.a c c/a H/f.u. a c c/a

RFe3 with PuNi3 structure type

YFe3 II 5.137 24.61 4.79 4.8 5.375 26.46 4.92 4.6 7.5 17.9 2.3 [14]

SmFe3 II 5.187 24.91 4.80 4.2 5.40 27.09 5.02 4.1 8.8 17.9 2.7 [18]

GdFe3 II 5.167 24.71 4.78 3.1 5.38 27.01 5.02 4.1 9.3 17.5 3.8 [19]

TbFe3 II 5.143 24.64 4.79 4.2 5.355 26.71 4.99 4.1 8.4 17.5 2.6 [19]

DyFe3 II 5.116 24.55 4.80 1.8 5.26 25.54 4.86 2.8 4.0 10.0 3.4 [20]

II 2.5 5.34 25.80 4.83 4.4 5.1 14.5 3.6 [20]

II 4.2 5.36 26.40 4.93 4.8 7.5 18.0 2.7 [20]

DyFe3 II 5.130 24.52 4.80 3.0 5.31 26.59 5.01 3.5 8.4 16.2 3.3 [19]

HoFe3 II 5.177 24.48 4.73 3.6 5.316 26.39 4.96 2.7 7.8 16.4 2.8 [19]

ErFe3 II 5.096 24.48 4.80 1.5 5.20 25.17 4.84 2.0 2.8 7.1 2.9 [21]

II 2.7 5.26 25.68 4.88 3.2 4.9 11.8 2.7 [21]

II 4.0 5.30 26.40 4.98 4.0 7.8 16.7 2.5 [21]

ErFe3 II 5.104 24.56 4.80 2.7 5.267 26.16 4.87 3.2 6.5 13.4 3.0 [19]

CeNi3 structure type

CeNi3 III 4.964 16.52 3.33 2.8 4.8748 21.590 4.43 ––1.8 30.7 27.7 5.9 [22]

b ¼ 8.5590 4.37 ––0.5

CeNi3 III 4.964 16.53 3.33 3.3 4.934 21.73 4.40 ––0.6 31.5 29.9 5.3 [11]

III 5.2 4.938 22.44 4.54 ––0.5 35.8 34.3 3.9 [11]

R2Ni7 with Ce2Ni7 type of structure

La2Ni7 III 5.059 24.68 4.89 6.5 4.9534 29.579 5.97 ––2.1 19.9 14.9 3.1 [23]

Ce2Ni7 III 4.941 24.51 4.96 4.4 4.9146 29.629 6.03 ––0.5 20.9 18.9 5.5 [24]

b ¼ 8.4651

III 4.7 4.9251 29.773 6.05 ––0.3 21.5 21.1 5.87 [24]

b ¼ 8.4933 6.07 ––0.8

Ce2Ni7 III 4.939 24.50 4.96 4.1 4.8845 29.607 6.07 ––1.3 20.9 19.1 6.0 [25]

b ¼ 8.507 6.02 ––0.3

La1.5Mg0.5Ni7 II 5.029 24.22 4.82 4.45 5.3854 26.437 4.91 7.1 9.1 25.2 3.8 [26]

II 4.55 5.3854 26.437 4.91 7.1 9.1 26.3 3.8 [26]

R2Co7 with Ce2Ni7 structure type

Ce2Co7 III 4.940 24.46 4.95 6.0 4.949 29.69 6.00 0.2 21.4 21.8 4.7 [14]

Pr2Co7 I 5.058 24.51 4.85 2.5 5.081 26.30 5.18 0.5 7.3 8.3 4.5 [27]

II 5.8 5.312 26.01 4.90 5.0 6.1 17.1 4.0 [27]

Nd2Co7 I 5.053 24.43 4.84 2.7 5.069 26.29 5.19 0.3 7.6 8.3 4.1 [27]

II 6.2 5.268 25.92 4.79 4.3 6.1 15.3 3.3 [27]

R2Co7 with Gd2Co7 structure type

Y2Co7 I 5.002 36.15 7.23 1.5 4.988 37.72 7.56 ––0.3 4.3 3.8 3.3 [27]

II 3.0 5.138 38.45 7.48 2.7 6.4 11.6 5.0 [27]

Gd2Co7 I 5.017 36.31 7.24 2.6 5.012 39.04 7.79 ––0.1 7.5 7.3 3.7 [27]

II 5.9 5.199 38.62 7.43 3.6 6.4 14.2 3.2 [27]

Tb2Co7 I 5.007 36.27 7.24 2.7 5.011 38.96 7.78 0.1 7.4 7.6 3.7 [27]

II 6.6 5.175 38.48 7.44 3.4 6.1 13.3 2.7 [27]

Dy2Co7 I 4.988 36.15 7.25 2.6 4.984 38.70 7.77 ––0.1 7.1 6.9 3.4 [27]

II 6.4 5.169 38.31 7.41 3.6 6.0 13.8 2.8 [27]

a: Types of hydrides: conventional interstitial hydrides: I – lower, II – higher; III –anisotropic hydrides.

sion of the unit cells. The named hydrides possess distinctcrystal chemistry features and belong to the group of ani-sotropic hydrides. Anomalously high specific volume ex-pansion per absorbed atom H, 5.3–6.0 �A3 significantly ex-ceeds these values for the isotropic hydrides.

Crystal chemistry of the hydrides belonging to groups Ior II contrasts to the group III, which exhibits a princi-pally different behaviour. Groups I and II are conven-tional, interstitial type hydrides, despite the hydrogenation

normally leads to an uneven expansion of the unit cells indifferent directions. However, formation of the hydridespreserves the coordination characteristics of the metalatoms with hydrogen atoms filling the interstitial sites inthe metal sublattice; thus, from a crystallographic point ofview they can be classified as formed via an isotropic me-chanism of hydrogenation. In contrast, a principally differ-ent mechanism of crystallographic transformation takesplace for the hydrides belonging to the type III where a


Table 2. Crystal structure data for the deutrerides of RT3 and R2T7 compounds from the powder neutron diffraction.

Hydride Spacegroup

a, �A c, �A DV=V(RT2)

DV=V(RT5)

Distance, �A D=RT2 D=RT5 Types of occupiedinterstices (Fig. 3)

Ref.R––D T––D

CeNi3-type of IMC structure

CeNi3D2.8 Pmcn 4.8748b ¼ 8.5590

21.590 58.1 ––1.8 2.19 1.48 4.04 0.22 1þ 2þ 4þ 7 [22]

CeNi3D3.3 P63/mmc 4.890 21.78 48.0 9.0 2.20 1.40 4.82 0.87 1þ 2þ 8 [11]

CeNi3D5.2 P63/mmc 4.902 22.34 46.3 18.9 2.10 1.43 7.23 1.68 1þ 2þ 8 [11]

Ce2Ni7-type of IMC structure

La2Ni7D6.5 P63/mmc 4.9534 29.579 56.6 0.0 2.39 1.52 5.00 1.50 1þ 2þ 7 [23]

La1.5Mg0.5Ni7D8.9 P63/mmc 5.3854 26.437 29.9 24.5 2.08 1.51 3.78 5.32 3þ 4þ 5þ 6þ 7þ 8 [26]

La1.5Mg0.5Ni7D9.1 P63/mmc 5.3991 26.543 29.6 22.9 2.06 1.48 3.53 5.32 3þ 4þ 5þ 6þ 7þ 8 [26]

Ce2Ni7D4.7 Pmcn 4.9251b ¼ 8.4933

29.773 62.1 ––0.6 2.07 1.53 4.08 0.58 1þ 2þ 3þ 7 [24]

Ce2Ni7D4.4 Pmcn 4.9146b ¼ 8.4651

29.629 59.8 ––1.0 2.20 1.53 3.96 0.46 1þ 2þ 3þ 7 [24]

Ce2Ni7D4.1 Pmcn 4.8845b ¼ 8.507

29.607 58.2 ––2.6 2.03 1.38 3.23 0.38 1þ 2þ 3þ 4þ 7 [25]

PuNi3-type of IMC structure

LaNi3D2.7 C2/m 8.6392 32.776 44.2 ––3.0 2.16 1.47 3.2 0.25 1þ 2þ 4þ 7 [6]

b ¼ 4.9265 48.8 0.2 4.2 0.28

b ¼ 90.850�

LaY2Ni9D12.8 R�33m 5.396 26.885 27.4 24.7 1.88 1.57 5.26 1.27 3þ 4þ 5þ 6þ 7þ 8 [12]

CeY2Ni9D7.7 R�33m 4.872 31.312 47.0 ––2.7 2.04 1.55 3.86 0.17 1þ 2þ 4þ 7 [12]

CeCo3D4 R�33m 4.961 32.69 63.9 0.4 2.21 1.51 5.81 0.47 2þ 7 [11]

CeCo3D6 R�33m 5.03 32.98 58.5 15.7 2.13 1.48 7.33 2.9 2þ 7þ 8 [11]

ErNi3D1.23 R3m 4.9718 25.901 15.1 1.0 2.26 1.64 1.85 0 5þ 6 [10]

ErNi3D1.97 R�33m 5.0456 26.157 18.4 6.4 2.16 1.50 2.75 0.42 5þ 6þ 7þ 8 [10]

ErNi3D3.75 R�33m 5.2398 26.605 24.3 22.4 1.72 1.47 4.93 1.51 4þ 5þ 6þ 7þ 8 [10]

ErNi3H3.7 R�33m 5.184 26.27 22.4 16.0 2.23 1.43 5.82 0.40 3þ 8 [11]

ErNi3H4.9 R�33m 5.21 26.45 23.0 19.5 2.38 1.53 7.18 1.08 3þ 8 [11]

ErCo3D1.37 R�33m 4.9837 26.057 16.1 ––1.0 2.20 1.65 2.05 0.00 5þ 6 [17]

ErCo3D3.71 R�33m 5.2218 26.046 24.3 11.9 2.01 1.64 5.17 0.83 3þ 5þ 7þ 8 [17]

ErCo3D3.4 R�33m 5.217 26.123 18.5 ––1.3 2.05 1.50 4.52 1.16 5þ 7þ 8 [15]

HoNi3D1.27 R3m 4.9887 26.097 13.2 4.6 2.07 1.63 1.91 0.00 5þ 6 [9]

HoNi3D1.8 R�33m 4.991 26.12 22.4 0.5 2.13 1.55 2.37 0.63 6þ 7 [28]

HoNi3D1.81 R3m 5.0351 26.297 16.1 –0.63 2.10 1.48 2.65 0.12 5þ 6þ 7þ 8 [9]

YCo3D1.3 R�33m 5.0209 25.957 13.2 –2.0 2.22 1.68 1.99 0 5 [16]

YCo3D2 R�33m 4.9992 26.93 21.0 13.1 2.33 1.69 3.00 0 5 [16]

YCo3D3.8 R�33m 5.241 26.401 28.8 24.7 2.35 1.62 5.04 1.51 5þ 7þ 8 [15]

YCo3D4.6 R�33m 5.2666 26.775 27.4 1.0 2.27 1.51 6.47 1.00 5þ 7þ 8 [16]

huge anisotropic change in structural parameters takesplace during the hydrogenation process.

Isotropic hydrides

Crystal structures of the hydrides belonging to the groupof isotropic hydrides were solved for the following materi-als:

YCo3D1.3�4.6 [15, 16] ; ErNi3D1.23�4.9 [10, 11] ;

ErCo3D1.37�4.1 [17], HoNi3D1.27�1.8 [9, 17] ;

LaY2Ni9D12.8 [12] ; La1.5Mg0.5Ni7D8.9�9.1 [26].

All of the R(Ni,Co)3 hydrides mentioned here are formedby IMC belonging to the PuNi3 type of structure. In total,the initial structure contains twelve types of tetrahedral sites(R2T2, RT3 and T4) (Fig. 2) and one type of octahedral(R2T4) site. The hydrogen sublattice in these hydrides isformed by a partial filling of nine from 13 available inter-stices, including 36i1, 36i2, 18h1, 18h2, 18h3, 18h5, 18h6,6c1 and 6c3. Particular structures include the following listand can be presented as filling of the sites mentioned below:

YCo3D1.3-2.0 � 36i1 Y2Co2 [16] ;

YCo3D3.8-4.6 � 36i1 Y2Co2 þ 18h2 Y2Co2 þ 36i2 YCo3

[15, 16] ;

ErNi3D1.23 � 36i1 Er2Ni2 þ 6c1 ErNi3 [10] ;

ErNi3D1.97 � 36i1 Er2Ni2 þ 6c1 ErNi3 + 36i2 ErNi3þ 18h2 Er2Ni2 [10] ;

ErNi3D3.75 � 36i1 Er2Ni2 þ 36i2 ErNi3 þ 18h2 Er2Ni2þ 18h3 Er2Ni2 + 6c3 Ni4 [10] ;

ErNi3D4.0-5.0 � 36i1 Er2Ni2 þ 36i2 ErNi3 þ 18h3 Er2Ni2[11] ;

ErCo3D4.1 � 36i1 Er2Co2 þ 18h2 Er2Co2 þ 36i2 ErCo3

[15] ;

ErCo3D1.37 � 36i1 Er2Co2 þ 6c1 ErCo3 [17] ;

ErCo3D3.71 � 36i1 Er2Co2 þ 36i2 ErCo3 þ 18h2 Er2Co2

þ 18h3 Er2Co2 [17] ;

HoNi3D1.8 � 18h1 Ho2Ni2 þ 6c1 HoNi3 [28] ;

HoNi3D1.3 � 36i1 Ho2Ni2 þ 6c1 HoNi3 [9] ;

HoNi3D1.8 � 36i1 Ho2Ni2 þ 6c1 HoNi3 þ 36i2 HoNi3þ 18h2 Ho2Ni2 [9] ;

LaY2Ni9D12.8 � 6c3 Ni4 þ 18h2 R2Ni2 þ 18h3 R2Ni2 þ18h6 RNi3 þ 36i1 R2Ni2 þ 6c4 Ni4 þ 18h5 RNi3þ 36i2 RNi3 [12] .

The only representative of isotropic hydrides formed byIMC with other than PuNi3 type structure studied so far isLa1.5Mg0.5Ni7D8.9�9.1 (Ce2Ni7 type) [26]. Nine types ofsites are filled by D in total, including tetrahedral(La,Mg)2Ni2, (La,Mg)Ni3, Ni4, tetragonal pyramidalLa2Ni3 and trigonal bipyramidal (La,Mg)3Ni2 interstices.Hydrogen is nearly equally distributed between the Laves-and Haucke-type slabs. The overall hydrogen contentcan be presented as LaMgNi4D7.56 (Laves-type) þ2 LaNi5D5.22 (Haucke-type) ¼ 2 La1.5Mg0.5Ni7D9.

Types of coordination of the hydrogen by metal atomsin the structures of both isotropic and anisotropic hydridesare shown in Fig. 3. Observed coordination includes octa-hedron R3Ni3 (1), tetrahedra R3Ni (2), R2Ni2 (3), Ni4 (4),RNi3 (6) and R2Ni2 (7), trigonal bipyramid R3Ni2 (5) andoctahedron R2Ni4 (8).

Crystal chemistry and metal-hydrogen bonding 679C

aCu 5

CaC

u 5M

gZn 2 36i1

Ho1

Ni236i2 Ni3

Ho2

Ni1

18h3

18h2

6c1

18h4

18h1

6c3

18h6

6c4

18h5

6c2

z

x y

Fig. 2. Potential sites for the accommodation of H atoms in the trigo-nal crystal structure of the PuNi3 type presented for R¼ Ho. 12 typesof the available tetrahedral interstices are shown as belonging to thetwo types of the stacking along [001] layers, Laves-type MgZn2 andHaucke-type CaCu5. These include 4 � Ho2Ni2 sites (18h1, 18h2,18h3 and 36i1), 5 � HoNi3 sites (36i2, 18h4, 18h5, 18h6 and 6c1) and3 � Ni4 sites (6c2, 6c3, and 6c4). Part of the unit cell from z ¼ 0 toz ¼ 1=3 is shown.

Fig. 3. Hydrogen coordination in the crystal structures of anisotropicand interstitial type hydrides.

The octahedron R3Ni3 (1) and tetrahedron R3Ni (2) areformed only in the structures of anisotropic hydrides, dur-ing their rebuilding. Other five types of interstitial sites (3,4, 6–8) can be occupied both in anisotropic and isotropichydrides; these sites already exist in the structures of theoriginal intermetallic alloys. Sites 1–6 are located insidethe Laves-type slabs; sites 7 and 8 belong to the CaCu5-type slabs.

Occupancies by hydrogen atoms of different sites inthe metal matrices are presented in Table 2.

During hydrogenation, interatomic Me––Me distancesin the conventional R(Ni,Co,Fe)3-based and R2(Ni,Co)7-based hydrides moderately increase. To illustrate this, wewill present the data describing transformations YCo3!YCo3D2.0 (type I)! YCo3D4.6 (type II) [16]. Hereexpansion of the unit cell proceeds to the extent ofallowing H atoms to reach equilibrium Me––H separa-tions; however, not changing significantly the coordina-tion polyhedra of the metals: 16-vertex polyhedronRR4(Ni,Co)12 for R atoms and 12-vertex polyhedron TT12

for Ni or Co (see Fig. 4). This expansion drasticallyweakens but does not break the bonding between transi-tion metal atoms compared with that between R and T; asexample, the Y––Co distances increase by 3.2–7.3%,while Co––Co ones are extended more significantly, by10.9–17.2%.

Thus, we conclude that hydrogenation does not signifi-cantly change the bonding mechanism which is dominatedby the metal-metal interactions with metal-hydrogen inter-actions playing much less significant role.

From analysis of the crystal structure data for theErNi3- and HoNi3-based hydrides [9, 10] it becomes evi-dent that these hydrides belong to the group of interstitialhydrides with H atoms filling 6 different types of inter-stices. Despite that the experimental diffraction pattern forall Er- and Ho-containing hydrides were satisfactorily de-scribed in the original group of symmetry R�33m, neverthe-less, for some of these hydrides an alternative descriptionwas suggested with loss of inversion symmetry and transi-tion to the space group R3m [9, 10]. Corresponding refine-ments yielded equally satisfactory results for both groups;however, description in the group R3m allowed avoidingshort H––H separations as a result of H ordering on thesplit sites. On further increase of the H content to 3.75 at.H/f.u. the formation of coordinated Ni atoms was ob-served. The H atoms in the vertices of these tetrahedra areshared with other neighbouring Ni having less than 4 Hneighbours. The proposal of ordering is reasonable; how-ever, absence of direct experimental proof calls for a sin-gle-crystal study of the hydrogenated materials to answerthe question concerning the symmetry of the unit cells andpossible H ordering.


Fig. 4. Hydrogenation-induced transformations of the 12-vertex coordination polyhedron R6T6 around the Co(Ni)1 atoms inside the MgZn2-typelayers in trigonal YCo3 (PuNi3 type) and hexagonal Ce2Ni7 structures. A two-step hydrogenation process leads to the formation of the “conven-tional” YCo3D2.0 and YCo3D4.6 hydrides; in contrast, an anomalously expanded “anisotropic” Ce2Ni7D4.7 is formed via a different mechanism.The figure shows characteristics of the lattice expansion illustrating that YCo3D2 hydride is formed with moderate expansion proceeding solelyalong the [001] direction, whereas during the formation of the higher YCo3D4.6 hydride further expansion proceeds in the basal plane. In theCe2Ni7D4.7 hydride an anomalously high expansion along the z axis takes place (>60%), with basal plane practically unchanged. The relevantdistances from the central atoms Co1/Ni1 to Y/Ce/Co/Ni atoms in their 12-vertexes coordination polyhedra are given in insets. For Co-containinghydrides, a rather modest elongation of the interatomic separations, up to 17.2% in maximum, does not change the original coordination charac-teristics. However, in case if Ce2Ni7D4.7 the situation is completely different. 3 from 6 Ni1––Ni distances undergo anomalously large increaseupon hydrogenation (by 71.3–72.6%), from the original 2.519 �A to 4.315–4.347 �A in the hydride phase, thus breaking the Ni––Ni interaction.Thus, coordination polyhedron charges to a 9-vertex Ni3Ce6 as a result of moving of three Ni atoms far away from the central atom Ni1.Hydrogen coordination of Co1 by six H atoms leads to the formation of octahedron Co1D6 with dCo1 . . . D¼ 1.69–1.70 �A. For Ni1, an open,saddle-type coordination Ni1D4 is observed with dNi1 . . . D ranging from 1.52 to 1.77 �A. This coordination is shown in the figure for both Co- andNi-containing compounds.

Interatomic distances R––H and T––H

Typical ranges of observed shortest interatomic distances(�A) are: Y––D (2.20–2.35); Ce––D (2.03–2.21); La––D(2.06–2.39); Er––D (2.04-2.26); Ho––D (2.10–2.13);Ni––D (1.43–1.63); Co––D (1.48–1.69). These values sa-tisfy a well known criterion dMe�H � rMe þ 0.25 �A, whererMe is the metallic radius of the metal atom.

Anisotropic hydrides

The hydrogenation mechanism is principally different forthe RT3 and R2T7 compounds of light rare earth elements,La and Ce, with Ni and Co formed via an anisotropicmechanism (type III). Crystal structure data for the aniso-tropic hydrides are available on the basis of the NPD ex-periments for CeNi3D2.8 [22], CeNi3D3.3–5.2 [11],LaNi3D2.8 [6], La2Ni7D6.5 [23], Ce2Ni7D4.7 [24],Ce2Ni7D4.4 [24], Ce2Ni7D4.1 [25], CeY2Ni9D7.7 [12] andCeCo3D4–6 [11].

Hydrogenation behaviour of RT3 and R2T7 compounds,formed by R ¼ La and Ce, substantially differs from thatof other isostructural hybrid intermetallics. It is charac-terised by an extremely strong anisotropic expansion ofthe unit cells proceeding along the [001] direction. Suchan expansion reaches values of Dc/c ¼ 30.7% for RT3

compounds and Dc/c ¼ 21.5% for R2T7 compounds; at thesame time the basal plane of the unit cells remains un-changed or even slightly contracts (see Table 1). Despitedifferences in types of the original structures (CeNi3,Ce2Ni7 and PuNi3-types) and type of T-element (Co orNi), similar anisotropic behaviour of lattice expansion isobserved for the compounds of light rare earth elements,La and Ce (also Y when alloyed with Ce [12]). The onlyexception is GdNi3H3.0 which also belongs to the type IIIhydrides [4].

Anisotropic lattice expansion in RT2/RT5

From Table 2 it is evident that in most cases anisotropicexpansion of hybrid structures proceeds within the RT2

slabs only leaving RT5 slabs without changes. Thisscheme is observed for all known systems, including RNi3(R ¼ La, Ce, Y0.67Ce0.33), CeCo3, R2Ni7 (R ¼ La, Ce), stud-ied under ambient hydrogen pressures. However, when hy-drogenation pressure increases to the level exceeding1 kbar, as in studies of the CeCo3––D2 and CeNi3––D2 sys-tems [11], further hydrogen uptake, despite keeping prefer-ence in expansion of the RT2 slabs, also involves a muchsmaller yet significant increase in the volumes of the RT5

layers. The values of the linear expansion are anomalouslylarge, reaching 35.8% for CeNi3D5.2 [11].

Changes in the metal-metal separationsand coordination polyhedra

A huge linear expansion along [001] causes a drasticchange in the metal sublattice within the RT2 slabs.

Figure 4 illustrates the deformation of the metal sublatticethat occurs inside the CeNi2 slabs of the Ce2Ni7 structureduring the D uptake [24]. Together with a huge expansionalong the [001] direction, substantial shifts of both thecentral Ni1 atom and surrounding Ce atoms are observed.As a result, the distances from the Ni1 atom to some ofthe formerly neighbouring Ni atoms increase by >70%(see Fig. 4). This dramatically changes its coordinationfrom 12 (Ce6Ni6) to 9 (Ce6Ni3).

Decrease of symmetry

Shifts of the R and T atoms inside the MgZn2-type slabsand hydrogen ordering cause deformation of the unit cellsand lowering of the initial hexagonal/trigonal symmetry tothe orthorhombic (CeNi3 and Ce2Ni7 types) or monoclinicone (PuNi3 type). Decrease of symmetry is manifested bythe appearance of extra, not allowed by the original hexa-gonal/trigonal structures, peaks in the SR XRD and PNDpatterns. From group–subgroup relations and observed ex-tinctions the symmetry of the hexagonal CeNi3 andCe2Ni7 was concluded to be reduced to an orthorhombicone in corresponding hydrides (P63/mmc! Cmcm!Pmcn). Similarly, for the LaNi3-based hydride, a monocli-nic unit cell was found (R�33m! P�33m1 ! C2/m). Devia-tion from the hexagonality is more pronounced for theCeNi3D2.8 (borth/

ffiffiffi

3p� aorth)/aorth � 1.4% [22]), compared

to Ce2Ni7D4.7 (borth/ffiffiffi

3p� aorth)/aorth � �0.5% [24]) and

Ce2Ni7D4.1 ((borth/ffiffiffi

3p� aorth) / aorth � 1.0% [25]). In the

latter system negative orthorhombic distortion observed forthe Ce2Ni7H4þ x sample saturated with hydrogen at pres-sure of 30 bar (borth/

ffiffiffi

3p� aorth)/aorth � �0.7% [25]) was

changed to a positive distortion reaching up to 1.0%,when the sample was in contact with air in an open glasscapillary. In case of LaNi3D2.8, amon/

ffiffiffi

3p� amon)/

bmon � 1.2%; b ¼ 90.85� [6]. As mentioned earlier in thispaper, a decrease of the symmetry from the space groupR�33m to the noncentrosymmetric R3m was suggested forsome of the ErNi3- and HoNi3-based hydrides includingErNi3D1.23 [10], HoNi3D1.27 and HoNi3D1.81 [9].

Hydrogen sublattice

The distribution of hydrogen in the structure of anisotropichydrides is very uneven – hydrogen atoms are accommo-dated only inside the MgZn2-type layers and on the bound-ary between Laves- and Haucke-type layers; the bulk ofthe Haucke-type slab remains empty. As a result, expan-sion of the anisotropic hydrides occurs only in MgZn2-typeslabs, whereas CaCu5-type slabs remain unchanged. Theexpansion of the RT2-slabs reaches 63.9% resulting inhuge anisotropic changes in the metal sublattice.

Because of such rearrangements, hydrogen atoms, insharp contrast to the known crystal structures of other in-termetallic hydrides, instead of filling initially existing in-terstices, attract cerium atoms into their surrounding andform new D-occupied sites, R3T3 octahedra and R3T tetra-hedra. This is illustrated by Table 3 where the data forCeNi3D2.8 and Ce2Ni7D4.7 hydrides are given. In addition,


three conventional interstitial type positions are occupied;these are tetrahedra R2Ni2 (2 types) and Ni4.

Hydrogen ordering

One of the important features of anisotropic hydrides isthe ordering of hydrogen atoms in the lattice with allH . . . H separations exceeding 1.8 �A. CeNi3D2.8 and

La2Ni7D6.5 are completely ordered whereas in Ce2Ni7D4.7

the part of the structure within the Laves-type slab is alsoordered. It is convenient to present the way the hydrogensublattice is organized by stacking of the coordinationpolyhedra formed by H atoms around Ce or La. In thecase of CeNi3D2.8, 12- and 7-vertex polyhedra of twotypes were identified [22] while for La2Ni7D6.5 a 15-vertexLaD15 polyhedron was formed. Stacking of these polyhe-dra allows building of H sublattice as layers filling theRT2 slabs.

Ni––H and Co––H interactions

Tetrahedral and open saddle-like NiH4 coordination, andCoH6 octahedra were observed in the crystal structures ofCeNi3D2.8 [22], Ce2Ni7D4.1 [25, 29], Ce2Ni7D4.7 [24],ErNi3D3.7 [10] and YCo3D2.0/4.6 [16]. Coordination of T byH increases from Ni (CN ¼ 4) to Co (CN ¼ 6, see Fig. 4).Interestingly, a different shape of Ni-H 4-fold coordinationwas reported in the same system, Ce2Ni7––D2, such as tet-rahedron NiD4 [25] or open saddle-type coordination NiD4

[24] (see Fig. 4). Later in this paper we will present theresults of the structural optimization based on densityfunctional total energy calculations performed forCe2Ni7D4.7 and Ce2Ni7H4.1. These calculations have con-firmed both the saddle-type and tetrahedral NiH4 coordina-tions and have shown that Ce2Ni7D4.7 is thermodynami-cally the more stable exhibiting the saddle type NiH4

environment.Unusual behaviours of anisotropic hydrides and com-

plexity of the metal-hydrogen interactions in these systemsraise questions about the nature of such interesting trans-formations from intermetallic alloy to a corresponding hy-dride.

Electronic structure and thermodynamicproperties

Total energies were calculated using the projected augment-ed plane-wave [30, 31] implementation of the Vienna abinitio simulation package [32, 33]. The generalized-gradi-ent approximation [34–36] was used to obtain accurateexchange and correlation energies for a particular config-uration of atoms. Ground-state geometries were deter-mined by minimizing stresses and Hellman-Feynmanforces with the conjugate gradient algorithm, until forceson all atomic sites were less than 10�3 eV �A�1. Experi-mentally known structural parameters were taken as astarting point and cell volume, cell shape, and atomic po-sitions were relaxed simultaneously in a series of calcula-tions with progressively increasing precision. A final highaccuracy calculation of the total energy was performedafter completion of the relaxations with respect to k-pointconvergence and plane-wave cut-off. Brillouin zone inte-grations are performed with a Gaussian broadening of0.1 eV during all relaxations. From the various sets of cal-culations it was found that for the CeNi3 and Ce2Ni7phases 18 � 18 � 6 and for CeNi3D2.8, Ce2Ni7D4.7 andCe2Ni7D4.0 phases 24 � 12 � 6 k-point mesh in the whole


Table 3. Types of interstices occupied in the structures of CeNi3D2.8

and Ce2Ni7D4.1.

Intermetallic compound Deuteride CeNi3D2.8 Ce2Ni7D4.7

New interstices formed via the rearrangement of existing interstices

Ni4 Ce3Ni3 D1 D2

CeNi3 Ce3Ni3 D2, D7 D1, D3

Formation of new interstice due to “buckling” of the MgZn2-typelayer

No interstice available Ce3Ni D3, D5 D5, D6

Ce2Ni2 tetrahedra on the boundary between CaCu5- and MgZn2-typeslabs and inside the latter one

R2Ni2 R2Ni2 D4, D8 D7, D8, D9

Ce2Ni2 Ce2Ni2 D4

Filling of Ni4 tetrahedra in the MgZn2-type layer

Ni4 Ni4 D6

Brillouin zone with a 600 eV plane-wave cut-off are suf-ficient to ensure optimum accuracy in the computed re-sults. The density of states calculations were performedusing the tetrahedron method with the Blochl corrections[37].

Structural optimizations were carried out in order tounderstand the reasons for the anisotropic expansion effectduring incorporation of H in the CeNi3 and Ce2Ni7 phases.Experimental structural information was used as input(model 1). In order to verify whether the experimentallyknown phase is the correct ground state structure, the ori-ginal starting structures presented in the P1 symmetry(model 2) and additional structural relaxation calculationswere performed. All structures were fully relaxed (mini-mization of force and stress); no constraints on the atomicpositions and unit cell parameters were applied. The opti-mized structures were subjected to the symmetry analysiswhich showed a successful convergence of the calculationresults to the experimental crystal structure. As it can beseen from the data presented in Table 4 (only the data forthe CeNi3-based hydride is given), the optimized atomicpositions and lattice parameters are in very good agree-ment with the experimental findings. In the hydrogenatedCeNi3D2.77 phase H atoms completely fill seven differenttypes of sites; in addition one extra position [H8 (4c) site]is partially, 30%, occupied by H. In order to simplify thecalculations, we have limited our considerations to thefully occupied sites only and excluded the last positionH8; thus, stoichiometry of the calculated compound corre-sponds to a slightly lower H/CeNi3 ratio of 2.67 comparedto the experimentally determined value of 2.77. Similar to

the CeNi3D2.8, in Ce2Ni7D4.7 hydride the H sublattice ismostly ordered. It contains six completely occupied sites,H1––H6 (from nine H sites in total) and three partially occu-pied sites (H7––H9). For simplicity we have assumed that Hfully occupies the H7 site with an experimental occupancyfactor of 0.47 and ignored the H8 and H9 (experimentaloccupancies <0.4) sites in the calculations. Because of theassumed vacancy of the two last sites, there is a small differ-ence in the hydrogen stoichiometry between experimentalNPD data, D/Ce2Ni7¼ 4.65, and calculations, H/Ce2Ni7¼ 4.50. The calculated atomic positional parametersagree very well with the experimental findings. The calcula-tions confirm highly anisotropic lattice distortion with hugeexpansion along [001] (Dc/c ¼ 30.7%) and a small contrac-tion in the basal plane (�Da/a ¼ 1.8). Also, in agreementwith the experimental data, crystal structure calculationscorrectly predicted anomalously large volume expansion onhydrogenation, 5.9 �A3/atom H for both CeNi3- and Ce2Ni7-based hydrides.

The two structurally characterised hydrides with lowerhydrogen content, Ce2Ni7D4.4 [24] and Ce2Ni7D4.0 [25],show different structures as compared to the saturated hy-dride Ce2Ni7D4.7 [24]. However, we have made theoreticalcalculations only for Ce2Ni7H4.0. These studies showedthat agreement between the experimental and theoreticalstructural data is less satisfactory compared to that for thesaturated Ce2Ni7D4.7 and CeNi3H2.8 hydrides at normalconditions. Indeed, the difference in the calculated volumeof the unit cell, �3.3% (Ce2Ni7H4.0) compared to the ex-perimentally observed value, noticeably exceeded such di-vergences for CeNi3H2.8 and Ce2Ni7D4.7 (�2.7 to �2.8%).


Table 4. NPD-based data and theoretically optimised crystal structure data for CeNi3D2.77.Space group Pmcn (No. 62).Lattice parameters: a ¼ 4.8748(3); b ¼ 8.5590(5); c ¼ 21.590(2) �A. T ¼ 300 K. Data recorded on the D1A diffractometer. Theory: a¼ 4.750,b ¼ 8.645, c ¼ 21.344 �A. H/CeNi3¼ 2.67.

Atoms Sites Neutron powder diffraction data Theoryx y z x y z

Ce1 4c 1=4 0.430(3) 0.2514(9) 1=4 0.4216 0.2504

Ce2 4c 1=4 0.378(2) 0.0575(6) 1=4 0.3800 0.0559

Ce3 4c 1=4 0.087(4) 0.9364(8) 1=4 0.0459 0.9263

Ni1 4c 1=4 0.755(1) 0.5297(4) 1=4 0.7527 0.5250

Ni2 4c 1=4 0.929(2) 0.3348(6) 1=4 0.92 0.3481

Ni3 4c 1=4 0.748(1) 0.2461(6) 1=4 0.7453 0.2422

Ni4 4c 1=4 0.086(1) 0.2555(5) 1=4 0.0905 0.2592

Ni5 4c 1=4 0.938(1) 0.1564(5) 1=4 0.9214 0.1566

Ni6 8d 0.005(2) 0.8219(10) 0.8392(3) 0.003 0.8288 0.8398

Ni7 8d 0.497(2) 0.1774(11) 0.3532(3) 0.499 0.1662 0.3580

D1 4c 1=4 0.726(2) 0.8894(8) 1=4 0.7542 0.8888

D2 4c 1=4 0.080(1) 0.1120(5) 1=4 0.0823 0.1100

D3 4c 1=4 0.916(1) 0.4962(7) 1=4 0.9106 0.4857

D4 4c 1=4 0.233(1) 0.6498(5) 1=4 0.2133 0.6571

D5 8d ––0.030(2) 0.169(1) 0.0094(4) ––0.025 0.1555 0.0103

D6 4c 1=4 0.771(3) 0.1007(5) 1=4 0.7775 0.1023

D7 4c 1=4 0.971(2) 0.4180(10) 1=4 0.9812 0.4098

D8a 4c 1=4 0.478(1) 0.1514(8) Vacant

a: Occupancy 0.30 (3).

Evaluations of the heat of formation for Ce2Ni7H4.0 andCe2Ni7H4.7 (thermodynamic data will be presented later inthis paper) showed a disagreement of theoretical calcula-tions with experimental stability of the Ce2Ni7H4.0 hydrideas compared to Ce2Ni7H4.7 (lower thermal stability forCe2Ni7H4.0 from theoretical study instead of experimen-tally observed increase of the thermal stability with de-crease of the H content in the hydride). A detailed studyis in progress aimed on comparison of the Ce2Ni7-basedhydrides. The results of this work will be published else-where. One reason for the observed disagreements forCe2Ni7H4.0 is the possible thermodynamically nonequili-brium state of the studied sample which had been exposedto air, thus affecting its properties by partial oxidation.

Nevertheless, common features were observed in theelectronic structures of all three theoretically studied hy-drides, CeNi3H2.8, Ce2Ni7H4.7 and Ce2Ni7H4.0. These simi-larities will be presented and discussed later in the paper.

From the characteristic features of the DOS one maybe able to rationalize (see, e.g., Ref. [38]) the chemicalbonding in CeNi3 and Ce2Ni7 and changes introduced inthe metal-metal bonding upon hydrogenation. To the bestof our knowledge no electronic structure calculations haveapparently hitherto been undertaken for the consideredphases. In general, both initial intermetallic alloys and allthree hydrogenated phases have a finite number (seeFig. 5) of electrons at the Fermi level (EF), which classi-fies them as metals. The metallic character of all thesephases mainly originates from the finite contributions tothe DOS at EF from the Ce-5d and Ni-3d electrons.

In this review we present only the total DOS forCeNi3H2.7, Ce2Ni7H4.5 and Ce2Ni7H4.0 (Fig. 5) as represen-tative with typical data for the PDOS for Ni and H inCeNi3H2.7 (Fig. 6) and Ce2Ni7H4.5 (Fig. 7). The detaileddata on the partial DOS will be published in the forthcom-ing publications. Normally, introduction of hydrogenmodifies the electronic structure of the host alloy by crea-tion of metal-hydrogen bonding states, shift of the Fermilevel, and change in the width of bands and/or modifica-tion of the lattice symmetry. One common feature in theelectronic structures of these hydrides is the occurrence ofthe H states at the bottom of the valence band (VB) (seeFigs. 6 and 7). The inclusion of the additional bonding H-sstates in the energy range �9 to �3 eV changed not onlythe corresponding portion of the DOS but also systemati-cally shifted the EF towards the unoccupied states in thenon-hydrogenated phases (see Fig. 5). It is interesting tonote that in the non-hydrogenated intermetallics the siteprojected DOS is almost similar. On the other hand, afterhydrogenation, they become significantly different fromeach other (please, compare PDOS of Ni1 (which isbonded with H) in Fig. 6 with that of Ni2 and Ni3 (thoseare not bonded with H) in Fig. 7), with a component atlow energies, around �9 to �8 eV present only for the H-bound Ni. Further to that, the VB widths for the Ni statesalso change depending on presence or absence of their Hin their surrounding. Indeed, in CeNi3 for all Ni states theVB widths are around 7 eV. Though in the hydrogenated


Fig. 5. Calculated total density of states for CeNi3, Ce2Ni7,CeNi3H2.8, Ce2Ni7H4.0 and Ce2Ni7H4.5. The Fermi level is set to zero.

Fig. 6. Partial DOSs of Ni1, H3, H5, and H6 forming the NiH4 tetra-hedra belonging to the . . . H––Ni––H––Ni . . . chains in the structure ofCeNi3H2.8. The Fermi level is set at zero energy and marked by thevertical dotted line; s-states are shaded. PDOS for Ni3 which is notbound with H are shown for comparison.

CeNi3H2.8 phase the VB width for Ni1, Ni2, Ni5, Ni6,and Ni7 atoms those are bound with H are almost thesame (around 9 eV); in contrast with them, VB widths aredrastically reduced to ca. 6.1 eV for Ni3 and Ni4 (notbound with H).

From PDOS data for H atoms, a clear interrelation be-tween type of the coordination of the H sites (see Table 3)and its electronic configuration is evident. In CeNi3H2.8

the H atoms have 4 different coordination characteristics,including two types of octahedra, Ce2Ni4 (H6) and Ce3Ni3(H1 and H2), and two types of tetrahedra, Ce2Ni2 (H4)and Ce3Ni (H3, H5, H7).

As example, PDOS data for the H atoms bound to Ni1are shown in Fig. 6. From this figure, we conclude thatthe most strongly bound are H atoms with the largestamount of Ni in their coordination. Indeed, for H6 with4 Ni/H, filled energy levels span from �9 to �5 eV with amaximum at �8 eV. For H1 and H2 with 3 Ni/at. H, thefilled levels are in the same range, �9 to �5 eV; however,the peaks on the electronic density of states spectra shifttowards the higher energies (see Fig. 6).

When number of Ni neighbours decreases to 2 Ni/at. H(for H4), further shift of the peak in the energy spectrumtowards higher energies takes place (peak is observed at�6 eV); once again the same range of the energies is cov-ered, from �9 to �5 eV.

For H3 and H5 with just one Ni/at. H the peaks of theDOS shift further towards higher energy appearing higherthan �5 eV. However, for H7 with a similar, Ce3Ni, envi-ronment, peak from its DOS is at lower energies, between�7 and �6 eV.

From the DOS data for Ce2Ni7H4.5 (see Fig. 7) it isclear that, similar to CeNi3H2.8, the bonding energy is alsorelated to the number of Ni atoms in the environment ofH; the strongest bonding takes place for 3Ni/at. H(H1––H3); the weakest bonding is observed for 1 Ni/at. H(H5 and H6), with 2 Ni/at. H (H4, H7) lying in between.Figure 7 presents the PDOS data for H6, H5, and H4forming an open saddle type configuration around Ni1. Aclear similarity between the structure of PDOS of H3 andH5 in CeNi3H2.8, H5 and H6 in Ce2Ni7H4.5, all with thesame, Ce3Ni, environment is evident from comparison ofthe Figs. 6 and 7.

The electron localization function (ELF) is consideredas a useful tool to distinguish different bonding interactionin solids (for more details about ELF see Refs. [39–42]).The value of ELF spans the range 0 to 1. A high value ofELF corresponds to a low Pauli kinetic energy, as can befound in covalent bonds or lone electron pairs. The largevalues of the ELF at the H site indicate strongly pairedelectrons with dominant s-electron character. From calcula-tions, it appears that ELF distribution on the H sites is notspherical indicating a finite covalent interaction of H withneighbours. We note that due to the presence of deloca-lized metallic Ni-d electrons, the calculated ELF on the Nisite is low. Table 5 presents summary of the PDOS char-acteristics of H atoms in the crystal structures of


Fig. 7. Partial DOSs of Ni1, H4, H5, and H6 forming an open sad-dle-type hydrogen configuration around Ni1 in the structure ofCe2Ni7H4.5 and belonging to the spatial . . . H––Ni––H––Ni . . . chains.The Fermi level is set at zero energy and marked by the verticaldotted line; s-states are shaded. PDOS for Ni2 which is not boundwith H are shown for comparison.

Table 5. Summary of the PDOS characteristics of H atoms in the crystal structures of Ce2Ni7H4.7, CeNi3H2.8 and Ce2Ni7H4. Three featuresreviewed include position of the center of the H band, its width, and integrated charge on the H atom.

Ce2Ni7H4.7 CeNi3H2.8 Ce2Ni7H4,1

Center of theband (eV)

Width(eV)

Integrated(e)


Width(eV)

Integrated(e)


Width(eV)

Integrated(e)

H1 ––7.5 3 0.53 ––6 5.5 0.48 ––7 4.45 0.52

H2 ––7.44 2.75 0.53 ––6.6 5.1 0.56 ––6.6 4.3 0.49

H3 ––6.5 2.65 0.56 ––5.4 2.48 0.55 ––4.7 5.2 0.54

H4 ––6.7 5 0.55 ––7 3.5 0.61 ––6 5.2 0.49

H5 ––4.5 3.2 0.54 ––4.8 2 0.58 ––5 2.72 0.56

H6 ––4.7 2.88 0.52 ––7.7 3.62 0.54 ––8 4.3 0.53

H7 ––6.6 4 0.56 ––4.5 5.3 0.45 ––6 6.0 0.47

Ce2Ni7H4.7, CeNi3H2.8 and Ce2Ni7H4. The absence of dif-ferences in the behaviour of H forming a tetrahedral or anopen saddle-type coordination around Ni and other typesof H in the materials worthy to mention. It should be alsomentioned that an integrated charge on the H atom variesin a rather narrow range from 0.47 to 0.61 e� and neverapproaches value of �1 necessary for the formation ofhydrido-ion H�1. For comparison, the Bader effectivecharges are calculated at the H sites in nearly pure ioniccases such as LiH and MgH2 and are -0.84 and �0.92,respectively. The much smaller value of the Bader effec-tive charge at the H sites for the systems considered in thepresent study clearly indicates that they do not reach apure ionic case of H�1.

We note that similar data concerning the Ni-H bondingin the clusters [Ni2H7] found during the studies of thecrystal structure of LaMg2Ni2H8 [43] were reported in[44]. Electronic configuration of these clusters [44] wasassumed as [Ni2H7]7� and showed absence of the forma-tion of H�1 and Ni0 and a partial negative charge on bothNi and H corresponding to the formula [Ni20.95�H7

0.73�]7�.In order to understand the microscopic origin of the

anisotropic volume changes during hydrogenation andlarge variation in lengths of the Ni––H, and Ce––H bondsin the system, we performed valence charge density ana-lyses in different crystal planes for the nonhydrogenatedas well as the hydrogenated phases. The electronegativitydifference between Ce and Ni is 0.8, which indicates thationic interaction between these atoms is most probable.This was indeed confirmed by the charge density analysis,which shows that charges in CeNi3 and Ce2Ni7 intermetal-lics are distributed in a spherically symmetric manner andthere is no finite electron density present between Ni andCe. In contrast, in the hydrides CeNi3D2.67 and Ce2Ni7H4.5

(charge density distribution is shown in Figs. 8a and b,respectively) Ni is bonded with H in a directional mannerindicating covalent type interaction. In contrast to the[Ni0H4

1�]4� complex, where hydrogen exists in a hydrido-

form holding a charge of �1, results from electronic struc-ture calculations show that a) further to a partial negativecharge on H, 0.5 to 0.6 e�, Ni is also carrying negativecharge reaching a maximum value of �0.3 and does nothave an 18-electron configuration formed in case of[Ni0H4

1�]4�; and b) a partial positive charge on Ce israther low, smaller than 1.5 and, obviously, far away fromCe3+ or Ce4+ configurations. From these observations weconclude that [Ni0H4

1�]4� complexes and Ce3þ/4þ ions arenot observed in CeNi3H2.8, Ce2Ni7H4.7 and Ce2Ni7H4.0. Inaddition to the small difference in electronegativity valuesbetween the Ni and H (only 0.3), the spatial and energeticdegenerate nature of electrons contributes to the covalentNi––H bonding interaction. Consequently, structural chains. . . H––Ni––H––Ni––H . . . are formed in the CeNi3H2.8,Ce2Ni7H4.7 and Ce2Ni7H4.0 hydrides. This feature is signif-icantly different from the behaviours of the RNiInH1.33

(R ¼ La, Ce, and Pr) phases where the formation ofdumbbell-like H––Ni––H structural subunits is observedcaused by strong Ni––H bonding; this also results in for-mation of very short H . . . H separations, around 1.6 �A[38]. Interestingly, interaction between Ni atoms is signifi-cantly different between the CeNi3 and CeNi3H2.8 phases.In the CeNi3 intermetallic alloy all Ni have metallic bond-ing and almost similar behaviour; in contrast, in CeNi3H2.8

hydrogen induces a difference in the bonding interactionbetween various Ni sites leading to shorter Ni––Ni con-tacts, stronger bonding part of these Ni atoms and conse-quent breaking of the bonds with the remaining Ni atomscausing huge anisotropic changes in the lattice.

In an effort to quantify the bonding interaction betweenatoms and estimate the amount of electrons on and be-tween the participating atoms we have made a Bader topo-logical analysis. Although there is no unique definition toidentify how many electrons are associated with an atomin a molecule or an atomic grouping in a solid, it hasnevertheless proved to be useful in many cases to performsuch analyses [45–47]. In the Bader charge (BC) analysis


a� b�

Fig. 8. Calculated charge density distributions inCeNi3H2.8 (a) and Ce2Ni7H4.5 (b) along the 101plane. The parts of the . . . H––Ni––H––Ni . . .chains are seen.

each atom in a compound is surrounded by a surface(called Bader regions) that run through minima of thecharge density and total charge of an atom is determinedby integration within the Bader region. The calculatedBC for the non-hydrogenated phases shows that Ce al-ways donated almost 1.2 electrons to the Ni sites (seeTable 6). Similarly, in the hydrogenated phases Ce do-nates 1.2 to 1.6 electrons to the host lattice, i.e. to Niand H sites. In the CeNi3H2.8 phase Ni at the Ni5-sitedoes not donate or accept electrons from the Ce sitesand, hence, calculated change in Bader effective charge(BEC, defined as the difference between atomic chargeand BC) for the Ni5 atoms between hydrogenated andnonhydrogenated phases is almost zero. The present cal-culations show that, in general, H always accepts 0.4 to0.6 electrons from Ni and Ce. The calculated BC indi-cates also that in the CeNi3H2.8 phase charges are trans-ferred from the CeNi5 layer to CeNi2 structural subunitsincreasing concentration of electrons in the latter unitsand, consequently, making it possible to accommodatemore H atoms in the Laves-type slabs.

Spatial chains ––H––Ni––H––Ni–– are formed in bothCe2Ni7H4.7 (Fig. 9a) and CeNi3H2.8 (Fig. 9b). These net-works contain terminal bonds Ni––H, bridges Ni––H––Ni,and the bonds where one H is bound to three different Ni.Central Ni atoms have the same CN 4, but different typeof coordination by hydrogen in these two materials viz.open saddle-type coordination NiH4 in Ce2Ni7H4.7

(Fig. 7a) and a tetrahedron NiH4 in the structure of

CeNi3H2.8. This differentiates anisotropic hydrides fromcomplex hydrides containing tetrahedral [Ni0H4

1�]4� ions,for example, from Mg2NiH4. However, such a differenceis not surprising as anisotropic hydrides are built via for-mation of spatial ordered frameworks between covalentlybonded Ni and H, whereas the dominant ionic bondinginteraction prevails in complex hydrides. Formation ofthese frameworks during the hydrogenation process causesrebuilding of the metal sublattice and this is a principalreason for the anisotropic changes in the structural para-meters on hydrogenation.

Finally, the heats of formation for the studied interme-tallic hydrides were theoretically calculated from the totalenergies obtained for the optimized systems. For theCeNi3- and Ce2Ni7-based hydrides, in spite of significantdifferences in the Ce/Ni and CeNi5/CeNi2 ratio of slabs,the experimentally determined heat of formation, DHH, isvery close, �22.4 and �22.6 kJ/mol H, respectively [24].Our calculations showed that theoretical values forCeNi3H2.8, �23.6 kJ/mol H and for Ce2Ni7H4.5, �27.4 kJ/mol H, are close to each other and well agree with theexperimental values. For Ce2Ni7H4.0, the calculated valueof enthalpy of formation is 5.6 kJ/mol H higher comparedto that in Ce2Ni7H4.5, indicating a lower stability of thetetrahydride Ce2Ni7H4.0 compared to the saturated hydrideCe2Ni7H4.7. This conflicts with the result anticipated fromthe PCT dependence [24] behaviour of the Ce2Ni7––H2

system, where stability of the hydride increases inCe2Ni7H4:7�x (x ¼ 0–0.7) with lowering of the H contentcompared to the saturated hydride Ce2Ni7H4.7. In addition,the PCT diagram does not show a predicted first orderphase transition in the desorption isotherm of theCe2Ni7––H2 system [25], thus raising a question of influ-ence of oxygen on the hydrogen evolution from the satu-rated hydride Ce2Ni7H4.7 thus modifying the behaviour ofthe whole system.

A very unusual alteration of the characteristics of thehydrogen interaction with the related Co- and Ni-contain-ing binary intermetallics, respectively, CeNi3 and CeCo3,Ce2Ni7 and Ce2Co7, was noted in [24]. This alteration isobviously caused by the preferential accommodation ofhydrogen by the RNi(Co)2 layers in the materials studiedhere which differs from the conventional interstitial-typeintermetallic hydrides. In the latter case Co-containing sys-tems are characterised by a higher thermal stability of thehydrides and, correspondingly lower values of enthalpiesof the hydrogenation DHH (see [24] where the data forsuch interstitial types, conventional CeNi5- and CeCo5-based hydrides are given). In contrast, an opposite beha-viours are observed for the Ni and Co equiatomic com-pounds forming anisotropic hydrides, Ce2Ni7–Ce2Co7 andCeNi3–CeCo3. These data clearly reflect the unusual beha-viours of the anisotropic hydrides. Further structural, theo-retical, and thermodynamic studies of the anisotropic hy-


Atom CeNi3 Ce2Ni7 CeNi3H2.8 Ce2Ni7H4 Ce2Ni7H4.5

Ce �1.2 �1.1 to �1.4 �1.3 to �1.6 �1.1 to �1.42 �1.1 to�1.5

Ni þ0.4 þ0.2 to þ0.4 0 to þ0.3 0 to þ0.28 0 to þ0.25

H þ0.4 to þ0.5 þ0.42 to þ0.46 þ0.4 to þ0.45

Table 6. Calculated average Bader charges(given in e) for CeNi3, Ce2Ni7, CeNi3H2.8,Ce2Ni7H4 and Ce2Ni7H4.5.

Ni

Ni1

DCe

DCe

Ni1

Ni

a�

b�

Fig. 9. . . . H––Ni––H––Ni . . . chains in the crystal structure ofCe2Ni7H4.7 (a) and CeNi3D2.8 (b).

drides will be of high importance to better understandthese unusual materials.

Conclusions

Hydrogenation of intermetallic alloys RT3 and R2T7

formed by rare earth elements R and transition metalsT ¼ Fe, Co, Ni proceeds via three different schemes ofinteraction and yields conventional interstitial hydrides,isotropic hydrides (types I and II), and anisotropic hy-drides (type III). From crystallographic point of view,these schemes have distinct differences in the way the in-termetallic structures are transformed into the hydrides:� Type I. Lower isotropic hydrides. Moderate expansion

of the original cells proceeds along [001] and giveshydrides RNi3H1.2–2.0, RCo3H1.3–2.1 and R2Co7H1.5–2.7

based on the RT3 and R2T7 intermetallics;� Type II. Higher isotropic hydrides. RNi3H3.4–4.3,

RCo3H3.6–4.6 and R2Co7H5.8–6.6 are formed via H up-take by the lower hydrides. Nearly similar lattice ex-pansion proceeds along [001] and in the basal plane.

� Type III. Anisotropic hydrides. Saturated hydrides ofLa and Ce, R(Ni,Co)3H2.7–5.2 and R2(Ni,Co)7H4.1–6.5,are formed by anomalously high linear expansion pro-ceeding exclusively along [001] and reaching 35.8%and yielding record high specific volume expansion(5.3–6.0 �A3) per absorbed H atom. Anisotropic hy-drides possess distinct crystal chemistry features dif-ferentiating them from isotropic ones.

Unique structural features observed during the formationof the anisotropic hydrides include: (a) Exclusive latticeexpansion within the RNi2 slabs. Such expansion, �60%along [001] for the Laves layers, is associated with occu-pancy by D atoms of these slabs; (b) D atoms inducedrearrangements of the metal sublattice and formation ofthe new types of interstitial sites; (c) Formation of an or-dered hydrogen sublattice.

DOS analysis of both Ce-containing intermetallicsCeNi3 and Ce2Ni7 and their corresponding hydridesshowed that these materials have common features in theirchemical bonding. This bonding in anisotropic hydrides ofCe with Ni may be described as having a mixed covalent-ionic type (Ce atoms donate 1.2–1.6 electrons to Ni andH), with H bonded to Ni by covalent bonding and Hbonded to Ce via ionic bonding. The bonding energy ofhydrogen atoms is clearly related to the Ni environment.The strongest bonding takes place for 4 Ni/at. H; theweakest bonding is observed for 1 Ni, with 2 Ni/at. H and3 Ni/at. H showing an intermediate behaviour. CDD andELF do not confirm the formation of the [Ni0H4

1�]4�

complexes in the hydrogenated phases. Instead, Ni iscovalently bonded with H resulting in the formation of�H––Ni––H––Ni–– spatial frameworks with different localcoordination of Ni by H (1, 2, 3 and 4 atoms). As a re-sult, local 4-fold coordination of Ni by H to form tetrahe-dra or a saddle-coordination Ni––4 H does not lead to aformation of the specific NiH4 units. This makes anisotro-pic hydrides completely different from the complex hy-drides containing [NiH4

4�] anions, where each atom H hasa bond with one Ni only.

The formation of ––H––Ni––H––Ni–– spatial chains isthe principal reason for the anisotropic changes in thestructural parameters on hydrogenation. The donation ofelectrons from nonhydrogenated RENi5 parts to hydrogenin RENi2 slabs stabilises these latter fragments.

Acknowledgments. We are grateful to the Research Council of Nor-way for the financial support and the computer time at the NOTURsupercomputer facilities.

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