R.V. Denys

Short hydrogen–hydrogen separations in novel intermetallic hydrides, RE3Ni3In3D4 (RE=La, Ce and Nd)

Abstract Crystal structure data for deuterides RENiInD x (RE=La, Ce and Nd) are provided on the basis of high-resolution powder X-ray and neutron diffraction data. The materials retain the hexagonal ZrNiAl type structure on deuteration. The formation of saturated deuterides is connected with anisotropic expansion along [001]. In the saturated hydrides, RE 3 Ni 3 In 3 D 4 , hydrogen atoms are located inside RE 3 Ni tetrahedra that share a common face, thereby forming a RE 3 Ni 2 trigonal bipyramid. This results in extraordinary short H–H separations of around 1.6 A. This feature is unique among well characterised metal hydride materials and is in striking contrast with the generally obeyed empirical rule of 2.0 A for H–H separations. On heating, the saturated materials release half of their hydrogen content at low temperatures, thereby statistically filling just one out of the two neighbouring tetrahedra. The remaining, more strongly bonded hydrogen, is released below 500°C under dynamic vacuum.

Hydrogen in La2MgNi9D13: The Role of Magnesium

Reversible hydrogen storage capacity of the La(3-x)Mg(x)Ni(9) alloys, charged by gaseous hydrogen or by electrochemical methods, reaches its maximum at composition La(2)MgNi(9). As (La,Mg)Ni(3-3.5) alloys are the materials used in advanced metal hydride electrodes in Ni-MH batteries, this raises interest in the study of the structure-properties interrelation in the system La(2)MgNi(9)-H(2) (D(2)). In the present work, this system has been investigated by use of in situ synchrotron X-ray and neutron powder diffraction in H(2)/D(2) gas and by performing pressure-composition-temperature measurements. The saturated La(2)MgNi(9)D(13.1) hydride forms via an isotropic expansion and crystallizes with a trigonal unit cell (space group R3m (No.166); a = 5.4151(1) Å; c = 26.584(2) Å; V = 675.10(6) Å(3)). The studied hybrid structure is composed of a stacking of two layers resembling existing intermetallic compounds LaNi(5) (CaCu(5) type) and LaMgNi(4) (Laves type). These are occupied by D to form LaNi(5)D(5.2) and LaMgNi(4)D(7.9). The LaNi(5)D(5.2) slab has a typical structure observed for all reported LaNi(5)-containing hybrid structures of the AB(5) + Laves phase types. However, the Laves type slab LaMgNi(4)D(7.9) is different from the characterized individual LaMgNi(4)D(4.85) hydride. This results from the filling of a greater variety of interstitial sites in the La(2)MgNi(9)D(13)/LaMgNi(4)D(7.9), including MgNi(2), Ni(4), (La/Mg)(2)Ni(2), and (La/Mg)Ni(3), in contrast with individual LaMgNi(4)D(4.85) where only La(2)MgNi(2) and Ni(4) interstitials are occupied. Despite a random distribution of La and Mg in the structure, a local hydrogen ordering takes place with H atoms favoring occupation of two Mg-surrounded sites, triangles MgNi(2) and tetrahedra LaMgNi(2). A directional bonding between Ni, Mg, and hydrogen is observed and is manifested by a formation of the NiH(4) tetrahedra and MgH(6) octahedra, which are connected to each other by sharing H vertexes to form a spatial framework.

Hydrides of the RNiIn (R=La, Ce, Nd) intermetallic compounds: crystallographic characterisation and thermal stability

LaNiInH2.0, CeNiInH1.8 and NdNiInH1.7 intermetallic hydrides were synthesised by the reaction of gaseous hydrogen with RNiIn compounds at 298 K and hydrogen pressures 1–100 bar and characterised by X-ray diffraction and thermal desorption studies. The hexagonal symmetry of the initial ZrNiAl-type structure is not changed on hydrogenation. Hydrogen insertion causes a pronounced anisotropic expansion of the unit cells along [001] (Δc/c=14.9–18.3%) and results in a volume increase of 8.9–9.3%. Possible interstitial sites for the accommodation of hydrogen atoms in the lattices of dihydrides RNiInH1.7–2.0 were proposed. A reversible formation of equiatomic RNiIn ternaries accompanies a complete hydrogen desorption from the dihydrides and takes place at temperatures near 800 K. Hydrogen evolution proceeds through two steps with peaks at 425–540 and 630–710 K and at temperatures 500–600 K leads to the formation of lower hydrides LaNiInH0.9, CeNiInH0.8 and NdNiInH0.85, which were structurally characterised as isotropically expanded ZrNiAl-type compounds. The melting points were determined for the LaNiIn (1057 K) and CeNiIn (1083 K) intermetallics. The NdNiIn compound exhibits high thermodynamic stability and does not disproportionate in hydrogen at PH2=1 bar up to 1023 K. RNiIn compounds formed with Y or the heavier rare earth metals (R=Sm, Gd, Tb, Dy, Ho, Er and Tm) do not form hydrides at hydrogenation pressures up to 100 bar, both at room temperature or on heating in hydrogen gas up to 1143 K.

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

Abstract Hydrides of the i3- and R2Ni7-based (R = light rare earth element) intermetallics exhibit novel structural features. Structures of these hydrides, including CeNi3D2.8, La2Ni7D6.5, LaNi3D2.8, and Ce2Ni7D4.7, are formed via a huge volume expansion occurring along a single crystallographic direction. Unique structural features during the formation of the hydrides include: (a) The lattice expansion proceeds exclusively within the RNi2 slabs leaving the RNi5 slabs unmodified. Such expansion, about 60% along [001] for the Laves layers, is associated with occupation by D atoms of these slabs; (b) New types of interstitial sites occupied by D are formed; (c) An ordered hydrogen sublattice is observed. In the present work we give (a) a review of the crystal chemistry of the conventional, interstitial type hydrides formed by RT3 and R2T7 intermetallic compounds (R = rare earths; T = Fe, Co, Ni) as compared to the unusual features of the crystal chemistry of anisotropic hydrides formed by the RNi3 and R2Ni7 intermetallics and (b) studies of the interrelation between structure and bonding in anisotropic hydrides by performing density functional calculations for CeNi3 and Ce2Ni7 intermetallic alloys and their corresponding hydrides. These studies allowed obtaining an understanding of the bonding mechanism in the hydrogenated compounds which causes a complete anisotropic rebuilding of their structures. From DOS analysis, both initial intermetallics and their related hydrides were found to be metallic. Bader topological analysis for the non-hydrogenated intermetallics showed that Ce atoms donate in average of almost 1.2 electrons to the Ni sites. Hydrogenation increases electron transfer from Ce; its atoms donate 1.2–1.6 electrons to Ni and H. Charge Density Distribution and Electron Localization Function for Ce2Ni7D4.7 phase clearly confirm that the interaction between the Ce and Ni does not have any significant covalent bonding. Ni is bonded with H via forming spatial frameworks –H–Ni–H–Ni– where H atoms accumulate an excess electron density of about 0.5e–. Thus, the tetrahedral or open saddle-type NiH4 coordination observed in the structures of the hydrides is not associated with the formation of [Ni0H41–]4– complexes containing a hydrido-ion H–1. In the structural frameworks there are terminal bonds Ni–H, bridges Ni–H–Ni, and the bonds where one H is bound to three different Ni. These spatial ordered frameworks stand as the principal reason for the anisotropic changes in the structural parameters on hydrogenation. Another unique feature of anisotropic hydrides is the donation of electrons from nonhydrogenated RNi5 parts to hydrogen in RNi2 slabs stabilising these fragments.

Interaction of hydrogen with RECu2 and RE(Cu,Ni)2 intermetallic compounds (RE=Y, Pr, Dy, Ho)☆

Hydrogenation of RECu2 (RE=Dy, Ho, Y) at room temperature and pressures of 100–1500-bar H2 has not resulted in the formation of ternary hydrides. The interaction of hydrogen with Pr(Cu1−xNix)2 (x=0, 0.1, 0.17, 0.25, 0.32) at room temperature and pressure of 25 bar resulted in the formation of Pr(Cu1−xNix)2H∼3 hydrides. It was found that the PrCu2H3 and Pr(Cu0.9Ni0.1)2H2.9 hydrides are poorly crystallized, but that an increase of the Ni-content leads to improved crystallinity of the hydrides. The hydrides Pr(Cu0.75Ni0.25)2H∼3 and Pr(Cu0.68Ni0.32)2H∼3 preserve, shortly after the hydrogen absorption, the CeCu2 type structure of their metallic matrix with a hydrogen induced volume expansion up to 28% compared to the parent compound. During long-term exposure in the air they undergo a structural transformation from the orthorhombic CeCu2 to the hexagonal Fe2P type with a hydrogen induced volume expansion up to 16.6% compared to the parent compound.

In situ powder neutron diffraction study of LaNiInD1.63 with short D…D distances

Abstract The recent powder neutron diffraction study of the crystal structure of LaNiInD 1.22 [J. Alloys Comp. 330–322 (2002) 132] concluded on the formation of a D…D pair with an unusually short interatomic distance of 1.63 A. Hydrogen atoms in LaNiInD 1.22 occupy a single crystallographic site and are coordinated by face-sharing La 3 Ni tetrahedra (92% occupancy). PCT measurements show that hydrogen storage capacity of LaNiIn, 1.63 at.H/formula unit, exceeds the limit of 1.33 at.H/LaNiIn when the La 3 Ni sites are completely occupied. In the present work, in situ powder neutron diffraction data were collected under D 2 pressure of 4.6 bar in order to study the deuteride with a maximum D content in the metal matrix. In the hexagonal structure of LaNiInD 1.63 (space group P 6 2 m ; a =7.3874(4); c =4.6816(2) A) Rietveld refinements showed that deuterium atoms occupy 36% of the available distorted La 3 NiIn 2 octahedra, in addition to the 96% filled La 3 Ni sites. The structure of LaNiInD 1.63 represents the first example of a deuteride containing direct In–D bonds (2.346(2) A). H bonding to the La 3 NiIn 2 sites is rather weak and a desorption from these sites takes place at room temperature and hydrogen pressures below 1 bar.

Powder neutron diffraction study of Nd6Fe13GaD12.3 with a filled Nd6Fe13Si-type structure

Abstract Nd6Fe13GaD12.3 with a filled Nd6Fe13Si-type structure has been synthesised from the corresponding intermetallic compound at room temperature under a D2 pressure of 1 bar and characterised by powder X-ray and high-resolution powder neutron diffraction (PND). Deuterium absorption gave a pronounced anisotropic lattice expansion along the c-axis [Δc/c:Δa/a∼11] with no change of the initial tetragonal symmetry [space group I4/mcm (No. 140); a=8.152(1); c=25.210(4) A]. Rietveld profile refinements of the PND data showed that deuterium atoms nearly completely occupy four different types of interstices, comprising Nd2Fe4 octahedra and three types of tetrahedra, Nd4, Nd3Fe and Nd2Fe2. The shortest metal–deuterium interatomic distances are: Nd–D=2.267 and Fe–D=1.71 A. Ga atoms are non-bonded to deuterium (all Ga–D separations >3.2 A). The deuterium sublattice is completely ordered with a shortest D–D distance of 2.248 A. It is built from two types of coordination polyhedra formed by D around neodymium atoms, [Nd1D5] and [Nd2D9], and can be described in terms of double layers of each type of polyhedra stacking along the [001] direction.

Zr4Al3D2.68 and Zr3Al2D2.26: new Zr-containing intermetallic hydrides with ordered hydrogen sublattice

Abstract Two hydrogenated intermetallics with the highest Al/Zr ratio among the hydrogen-absorbing Zr–Al compounds, Zr 4 Al 3 and Zr 3 Al 2 , have been studied by synchrotron X-ray, powder neutron diffraction and thermal desorption spectroscopy. Initial intermetallic compounds are quite different with respect to Al–Al interactions and contain plain Kagome Al nets (Zr 4 Al 3 ) or Al–Al pairs (Zr 3 Al 2 ). In hexagonal Zr 4 Al 3 D 2.68 (space group (s.g.) P 6 3 22; a =11.0017(4); c =11.1694(5) A) a 2 a ×2 a ×2 c superstructure is formed as a result of deuterium ordering in half of the available Zr 4 tetrahedra. These tetrahedra share common corners and edges and form layers separated by 6363 Al-nets. In tetragonal Zr 3 Al 2 D 2.26 (s.g. P 4 2 / mnm ; a =7.5970(3); c =7.2613(3) A) in addition to the completely filled Zr 4 tetrahedra hydrogen partially occupies Zr 3 triangular sites. Thermal stability of the studied deuterides and Zr–D bonding characteristics can be related to the size of the occupied Zr 4 tetrahedra. Higher thermal stability of Zr 3 Al 2 D 2.26 agrees well with the existence of large Zr 4 sites and contrasts to the behavior of Zr 4 Al 3 D 2.68 containing ‘contracted’ Zr 4 tetrahedra and having weaker Zr–D bonds.

Nanostructured Metal Hydrides for Hydrogen Storage Studied by In Situ Synchrotron and Neutron Diffraction

This work was focused on studies of the metal hydride materials having a potential in building hydrogen storage systems with high gravimetric and volumetric efficiencies of H storage and formed / decomposed with high rates of hydrogen exchange. In situ diffraction studies of the metal-hydrogen systems were explored as a valuable tool in probing both the mechanism of the phase-structural transformations and their kinetics. Two complementary techniques, namely Neutron Powder Diffraction (NPD) and Synchrotron X-ray diffraction (SR XRD) were utilised. High pressure in situ NPD studies were performed at D 2 pressures reaching 1000 bar at the D1B diffractometer accommodated at Institute Laue Langevin, Grenoble. The data of the time resolved in situ SR XRD were collected at the Swiss Norwegian Beam Lines, ESRF, Grenoble in the pressure range up to 50 bar H 2 at temperatures 20-400°C. The systems studied by NPD at high pressures included deuterated Al-modified Laves-type C15 ZrFe 2-x Al x intermetallics with x = 0.02; 0.04 and 0.20 and the CeNi 5 -D 2 system. D content, hysteresis of H uptake and release, unit cell expansion and stability of the hydrides systematically change with Al content. Deuteration exhibited a very fast kinetics; it resulted in increase of the unit cells volumes reaching 23.5 % for ZrFe 1.98 Al 0.02 D 2.9(1) and associated with exclusive occupancy of the Zr 2 (Fe,Al) 2 tetrahedra. For CeNi 5 deuteration yielded a hexahydride CeNi 5 D 6.2 (20°C, 776 bar D 2 ) and was accompanied by a nearly isotropic volume expansion reaching 30.1% (∆a/a=10.0%; ∆c/c=7.5%). Deuterium atoms fill three different interstitial sites including Ce 2 Ni 2 , Ce 2 Ni 3 and Ni 4 . Significant hysteresis was observed on the first absorption-desorption cycle. This hysteresis decreased on the absorption-desorption cycling. A different approach to the development of H storage systems is based on the hydrides of light elements, first of all the Mg-based ones. These systems were studied by SR XRD. Reactive ball milling in hydrogen (HRBM) allowed synthesis of the nanostructured Mg-based hydrides. The experimental parameters (P H2 , T, energy of milling, ball / sample ratio and balls size), significantly influence rate of hydrogenation. The studies confirmed (a) a completeness of hydrogenation of Mg into MgH 2 ; (b) indicated a partial transformation of the originally formed -MgH 2 into a metastable -MgH 2 (a ratio / was 3/1); (c) yielded the crystallite size for the main hydrogenation product, -MgH 2 , as close to 10 nm. Influence of the additives to Mg on the structure and hydrogen absorption/desorption properties and cycle behaviour of the composites was established and will be discussed in the paper.

Microstructure and hydrogen storage properties of as-cast and rapidly solidified Ti-rich Ti–V alloys

Abstract The goal of the present work was to optimize the phase-structural composition and microstructure of binary Ti 0.8-0.9 V 0.2-0.1 alloys with respect to their hydrogen sorption properties. Application of these alloys is for hydrogen absorption from gaseous mixtures containing substantial amounts of carbon monoxide (CO) at high temperatures. Irrespective of alloy composition, both α (HCP) and β (BCC) phases in Ti 0.8-0.9 V 0.2-0.1 formed single phase FCC hydrides upon hydrogenation in pure H 2 . An in situ synchrotron X-ray diffraction study showed that only the β -phase transformed to the corresponding hydride when the alloy was hydrogenated in a mixture of H 2 +10%CO. Rapid solidification (RS) of the alloy resulted in refined grain sizes both in the Ti 0.8 V 0.2 and Ti 0.9 V 0.1 alloys. Furthermore, RS was found to increase the β -phase fraction in Ti 0.9 V 0.1 , being twice larger than that of the as-cast alloy. Ti 0.9 V 0.1 had a platelike microstructure as observed by scanning electron microscopy (SEM), the plates were about 300 nm thick. The microstructure refinement resulted in a faster kinetics of H desorption as observed by temperature desorption spectroscopy (TDS).