V.A. Yartys

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.

Hydrogen absorption–desorption, crystal structure and magnetism in RENiAl intermetallic compounds and their hydrides

Abstract At ambient pressures RENiAl (RE=Y, Gd, Tb, Dy, Er and Lu) ternary intermetallic compounds crystallising in the ZrNiAl type of crystal structure form hydrides containing up to 1.4 H/f.u. Hydrogenation leads to a drastic reduction of magnetic ordering temperatures. It is accompanied by changes of magnetic structures and, for the majority of compounds, by orthorhombic distortion of the initial hexagonal symmetry.

Deuterofullerene C60D24 studied by XRD, IR and XPS

Abstract The deuterofullerene C60D24 was prepared from the solid C60 and was characterised by means of XRD, IR and XPS. The C60D24 was found to be a polycrystalline powder with a FCC lattice and a=14.55 A. Deuterium thermal desorption from C60D24 leads to a reversible formation of fullerene. However, the distances between the C60 molecules become significantly longer compared to the initial fullerite.

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, 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.

RNiAl hydrides and their magnetic properties

Abstract R NiAl compounds absorb hydrogen at normal pressure typical to a stoichiometry of R NiAlH = 1.4 . Thermal desorption studies showed that, besides such hydrides, several partly decomposed hydrides with lower H concentrations are also stable. The X-ray analysis shows that, in most cases, hydrogenation leads to an orthorhombic distortion of the original hexagonal structure of the ZrNiAl type. The hydrogenation severely affects magnetic properties depressing ordering temperatures and changing all the magnetic phase diagrams.

Further studies of hydrogenation, disproportionation, desorption and recombination processes in a Nd5Fe2B6 boride

Abstract The hydrogen absorption and desorption characteristics of, Nd5Fe2B6 were studied in this work, particularly at high temperatures where the hydrogenation, disproportionation, desorption and recombination (HDDR) process takes place. Two events of hydrogen absorption were observed for the ρ-Nd5Fe2B6 compound when heated in hydrogen (p(H2)=1 bar). In the lower temperature region (from 335 to 745°C) the formation of an insertion type rhombohedral hydride Nd5Fe2B6Hx was observed, leading to a rather small anisotropic lattice expansion (up to 2.63 vol%) in the [001] direction. At temperatures higher than 745°C, a disproportionation of this hydride occurs resulting in the formation of NdH3−x, NdB4 and η-Nd1.1Fe4B4. The recombination of the three phase mixture proceeds readily on heating in vacuum, employing conditions similar to those applied in the HDDR route for Φ-Nd2Fe14B.