A.B. Riabov

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.

Neutron diffraction studies of Zr-containing intermetallic hydrides with ordered hydrogen sublattice. I. Crystal structure of Zr2FeD5

Abstract The deuteration of intermetallic Zr 2 Fe with CuAl 2 -type structure was studied by Thermal Desorption Spectroscopy, powder X-ray and neutron diffraction. The tetragonal crystal structure of the saturated (1 bar D 2 ) Zr 2 FeD 5 deuteride (space group P4/ncc (No.130); 298 K: a =6.93566(8), c =5.62061(8) A; 4.2 K: a =6.92112(7), c =5.62045(7) A) has a completely ordered hydrogen sublattice both at 4.2 and 298 K. All interatomic D–D distances exceed 2.08 A. The crystal structure was determined by Rietveld analysis of high resolution powder neutron diffraction data. The presence of small amounts of three (four) impurity phases was included in the refinements. The D-sublattice is built from distorted tetragonal antiprisms of ZrD 8 , and can be described in terms of layers altering along [001]. Deuterium atoms occupy two types of tetrahedral Zr 4 and Zr 3 Fe interstices. Interatomic bond distances are in the range: Zr–D 2.055–2.136 A (298 K), 2.052–2.134 A (4.2 K); Fe–D 1.662 A (298 K), 1.658 A (4.2 K). The crystal structure is isotypic with Zr 2 CoD 5 . The unit cell expansion on deuteration and on heating from 4.2 to 298 K is highly anisotropic. The c -axis remains unchanged and preferable expansion in the a – b plane is discussed as connected to characteristics of metal–metal bonding and deuterium–deuterium repulsive interactions. The hydrogenation increases the stability of the CuAl 2 -type metal matrix of Zr 2 Fe. In the presence of interstitial hydrogen, the phase is stable far below the temperature of peritectoid decomposition of pure Zr 2 Fe.

Neutron diffraction studies of Zr-containing intermetallic hydrides with ordered hydrogen sublattice. II. Orthorhombic Zr3FeD6.7 with filled Re3B-type structure

Zr3FeD6.7 was obtained by deuteration of the Zr3Fe binary intermetallic compound with Re3B-type structure at temperatures near 273 K and D2-pressures below 0.25 bar, and was characterised by Thermal Desorption Spectroscopy and powder X-ray diffraction. Crystal structure data (new structure type of intermetallic hydrides) were derived at 7 K and 293 K from Rietveld refinements of high-resolution powder neutron diffraction data. The orthorhombic symmetry of Zr3Fe (space group Cmcm) is retained during hydrogen absorption; however, there is an anisotropic expansion in the unit cell dimensions. At 293 K a=3.5803(3); b=11.059(1); c=9.6486(8) A (Δa/a=7.7%, Δb/b=0.8%, Δc/c=9.4%). The deuterium atoms take an ordered structure with near complete filling of four different types of interstices, one trigonal bipyramidal Zr3Fe2 interstice and three types of tetrahedral interstices of Zr3Fe (one) and Zr4 (two). Metal–deuterium bond distances are in the range Zr–D=2.058–2.204 A, Fe–D=1.713–1.791 A (293 K). All D–D distances in the completely ordered hydrogen sublattice exceed 2.01 A. The sublattice is built from two types of polyhedra which surround zirconium atoms, consisting of deformed cubes with an additional ninth vertex [Zr1D9] and deformed cubes [Zr2D8]. These polyhedra form a spatial framework by sharing vertexes and edges. There are no indications for magnetic long range order in the powder neutron diffraction data at 7 K. A multistaged deuterium desorption starts just above room temperature for Zr3FeD6.7. At 448 K the less saturated deuteride Zr3FeD∼5, with a reduced unit cell volume, is obtained. Deuteration, when performed at 873 K and 1 bar D2, results in disproportionation of Zr3Fe into ZrD2 and ZrFe2. Both for Zr3FeD6.7 and the disproportionated phase mixture, deuterium desorption under secondary vacuum conditions is completed below 973 K and results in a full recovery of the Zr3Fe intermetallic. Zr3Fe presents a new example of successful application of the Hydrogenation–Disproportionation–Desorption–Recombination process to intermetallic phases of zirconium.

Neutron diffraction studies of Zr-containing intermetallic hydrides with ordered hydrogen sublattice, III. Orthorhombic Zr3FeDx (x= 1.3, 2.5 and 5.0) with partially filled Re3B-type structure

Abstract Lower deuterides of Zr 3 FeD x ( x =1.3, 2.5 and 5.0) were obtained by desorption of deuterium from Zr 3 FeD 6.7 and were studied by means of powder X-ray and neutron diffraction. Their metal sublattices are of the Re 3 B-type (space group Cmcm ; a =3.3261(4)–3.4524(1); b =11.2333(5)–11.317(1); c =8.999(1)–9.4746(4) A for x =1.3–5.0), and are hence mainly unchanged with respect to the initial intermetallic Zr 3 Fe and the “highest” deuteride Zr 3 FeD 6.7 . A gradual expansion of the orthorhombic unit cells accompanies the increase in D content. There appears to exist a broad solid solution phase, and the volume increase is in the range 4.8( x =1.3)–14.2%( x =5.0) relative to Zr 3 Fe. The expansion is anisotropic, for low x mainly located in the bc plane, for larger x in the ac plane. Rietveld refinements of the high resolution powder neutron diffraction data show that the initially occupied four types of interstices in Zr 3 FeD 6.7 , i.e. Zr 3 Fe 2 , Zr 3 Fe and two non-equivalent Zr 4 sites, are depopulated in a step-wise manner on desorption at increasing temperatures. On decreasing the D content, deuterium is first removed from interstices having iron atoms in their surroundings (Zr 3 Fe 2 and Zr 3 Fe sites), whereas the occupancy of the Zr 4 tetrahedra remains complete. In the “lower” deuterides, Zr 3 FeD 2.5 and Zr 3 FeD 1.3 , these tetrahedra become differently occupied. The reduced stability of one of these sites correlates with having an unfavourable smaller size. On going from Zr 3 FeD 6.7 to Zr 3 FeD 5.0 the originally completely ordered hydrogen sublattice with all D–D distances exceeding 2.0 A becomes partially disordered. The partially filled Zr 3 Fe interstices (50% occupancy) become closer on average (in Zr 3 FeD 5.0 : 1.54 A), which must be understood in terms of significant short range order. The metal–deuterium distances decrease gradually with decreasing D/Zr 3 Fe ratio, being shorter than those of Zr 3 FeD 6.7 : Zr–D=2.058(5)–2.204(7) A; Fe–D=1.713(4)–1.7914(3) A (at 293 K). There are no indications for either D ordering or for magnetic long range order in the powder neutron diffraction data of Zr 3 FeD 5.0 at 7 K.

Hydrogenation behaviour, neutron diffraction studies and microstructural characterisation of boron oxide-doped Zr–V alloys

Abstract Compositions in the range Zr 3 V 3 B 0.12–0.40 O 0.18–0.60 from the Zr–V–B 2 O 3 system have been subjected to metallographic characterisation, microprobe analysis and powder neutron diffraction. On melting, boron was found to be reduced from its oxide and in the annealed condition, it was identified as a constituent of two phases, η-oxyboride Zr 3 V 3 (B,O) and vanadium boride V 3 B 2 . Hydrogen absorption–desorption properties were studied and related to the phase and structural composition of the alloys. A redistribution of the light atoms (oxygen, boron) within the η-oxyboride Zr 3 V 3 (B,O) matrix and, also, between the constituent phases of the alloys takes place during high temperature cycling in hydrogen which could indicate increased lattice mobility of these non-metallic elements in the hydride material.

Hydrogen absorption and phase structural characteristics of oxygen-containing ZrV alloys substituted by Hf, Ti, Nb, Fe

Abstract Hydrogen absorption properties of ZrVO alloys where some of the Zr or V is substituted by Ti, Hf, or Nb, Fe, respectively have been investigated. Phase structural characteristics of these alloys and their hydrides have been determined by X-ray powder diffraction. The interdependence of obtained crystallographic and hydrogenation characteristics has been discussed.

(Hf, Zr)2Fe and Zr4Fe2O0.6 compounds and their hydrides : phase equilibria, crystal structure and magnetic properties

Abstract Hydrogen absorption and desorption properties of the (Hf,Zr) 2 Fe quasibinary alloys (Ti 2 Ni structure type) and the oxygen-stabilised compound Zr 4 Fe 2 O 0.6 with the filled-Ti 2 Ni type of structure, have been studied and discussed in relation to their phase-structural and chemical composition. The crystallographic characteristics, hyperfine parameters and magnetic properties of the hydrogenated alloys have been determined.

Oxide-modified ZrFe alloys: thermodynamic calculations, X-ray analysis and hydrogen absorption properties

Abstract To predict the possibility of formation of η-Zr 4 Fe 2 O x ternary intermetallics of Ti 2 Ni type structure during interaction between ZrFe melt and added oxide, the thermodynamic calculations for ZrFeR x O y systems were done. The influence of oxide addition on the hydrogen absorption properties of Zr 2 Fe alloys was studied.

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.

Hydrogen ordering and H-induced phase transformations in Zr-based intermetallic hydrides

Abstract Crystal chemistry aspects of hydrogen behaviour in the zirconium–iron intermetallic deuterides (hydrides), Zr2FeD1.80–5.00, Zr3FeD1.27–6.70 and Zr4Fe2O0.6H7.80, were studied with a focus on the application of high resolution powder neutron diffraction. The effects of crystal structure, chemical composition of the metal matrices, temperature and hydrogen contents on preferences in the interstices occupation and H ordering were investigated and discussed in relation to the H absorption–desorption properties. The Hydrogenation–Disproportionation–Desorption–Recombination process was successfully applied to all materials studied, including the first reported example of an oxygen-containing compound, Zr4Fe2O0.6.