C Lartigue

Neutron and X-ray diffraction profile analyses and structure of LaNi5, LaNi5−xAlx and LaNi5−xMnx intermetallics and their hydrides (deuterides)☆

Abstract LaNi 5 , LaNi 5−x Al x , LaNi 5−x Mn x and the deutendes of these intermetallics were investigated by neutron and X-ray diffraction. Nickel is replaced by manganese on both the 2c and 3g sites but by aluminium only on the 3g site. Contrary to previous structure refinements, the deuterides are described by a five-site structural model with the space group P6 mmm . The presence of aluminium prohibits occupation of three of the five interstices found to be occupied in LaNi 5 D 6.6 . Anisotropie line broadening of the activated compounds, which is attributed to particle size and microstrain effects, is reduced by both aluminium and manganese.

Correlations between the structural properties, the stability and the hydrogen content of substituted LaNi5 compounds

Thermodynamic and structural properties including electrochemical behaviour of LaNi5 and its β hydride are reviewed. The different structural models of the β hydride are discussed. The variations in the hydriding properties as a result of partial substitution of the constituents are presented. Qualitative and quantitative models that relate hydriding characteristics (stability, hydrogen content) to other physical properties (formation enthalpy, unit cell volume, interstitial hole size, electronic structure) of these intermetallics are briefly reviewed. The contribution of the authors concerning the correlation between the stability and the heat of formation of the intermetallic compounds is presented. The crystal structures of some substituted LaNi5 compounds and related hydrides demonstrate the correlation between the absorption capacity and the number of sites occupied by hydrogen atoms.

A new study of the structure of LaNi5D6.7 using a modified Rietveld method for the refinement of neutron powder diffraction data

Abstract The Rietveld profile analysis method has been extended using a Fourier analysis of line profiles on the basis of the Warren-Averbach method for separating size and strain effects. Assuming a given size distribution (Cauchy) and an adjustable strain variation in space, this method allows the simultaneous determination of the structural parameters and the size and strain parameters of the sample. It has been used for analysing the neutron diffraction pattern of β-LaNi 5 deuteride which exhibits a strongly anisotropic broadening. The mean coherency domain was chosen as an ellipsoid of revolution along the c axis. The structure is described in the space group P 6 3 mc as well as the P 31 c space group with five types of interstitial sites (three in the hypothetical fully ordered LaNi 5 D 7 phase). The strain parameter, i.e. the distribution of atomic positions around their equilibrium positions, varies by a factor of 2.5 while the mean particle size changes from 80 A to more than 2500 A in [ hk 0] and [00 l ] directions respectively.

Thermodynamic and structural properties of LaNi5−xMnx compounds and their related hydrides

Abstract Metallurgical, structural and thermodynamic properties of LaNi5−xMnx compounds and their related hydrides were determined. The limit of existence of pseudo-binary intermetallic compounds with the hexagonal structure P6/mmm is x = 2.2 at 800 °C. The lattice parameters and the cell volume increase with increasing manganese content, the change being linear up to x = 1.5. The hydrogen absorption and desorption isotherms were determined by an incremental method between 25 and 200 °C. The partial substitution of nickel by manganese involves a large decrease of the plateau pressure which is proportional to the manganese content. Neutron diffraction measurements show that manganese atoms mainly occupy the 3g sites in the LaNi5 structure and that the deutendes retain the same structure in which, although deuterium atoms are widely distributed on interstitial sites, there is a preference for the 6m and 12n sites.

Hydrogen (deuterium) ordering in the β-LaNi5Dx > 5 phases: A neutron diffraction study☆

Abstract A new investigation of the structure of the β-LaNi5Dx deuteride has been performed using neutron diffraction over a wide range of deuterium concentration (5.0 ⩽x ⩽ 6.7). By increasing the deuterium content, we show that the deuteride undergoes a structural phase transformation due to a progressive ordering of D atoms. This results in a reduction of symmetry and in a superstructure which corresponds to a doubling of the LaNi5 unit cell along the c axis. We propose a new model for the structure of β-LaNi5Dx (5.0 ⩽ × ⩽ 7.0) which is described in the P63mc space group. In this model the ordering of deuterium is characterized by a parameter a which varies linearly from zero to one as x increases from 5 to 7 D per LaNi5. In the fully ordered LaNi5D7 phase deuterium would occupy four types of interstitial sites.

In situ neutron powder diffraction measurements of the absorption and desorption of hydrogen (deuterium) in (La,Ce)Ni2 compounds: Amorphization and recrystallization

Abstract The structural transformation of RNi2 Laves phases (≡ La, Ce) on absorption of hydrogen and deuterium was studied using X-ray powder diffraction at room temperature, 90 and 120 °C and using time-resolved neutron powder diffraction at room temperature. For the intermetallic compounds, superlattice lines indexed in a double cubic cell are observed in addition to the diffraction peaks of the cubic C-15-type structure. In the case of in situ absorption measurements, small increases in the lattice parameters of the crystalline phases LaNi2.18 and CeNi2.16 are observed and the formation of an amorphous phase is detected from the first stage of the absorption. Under equilibrium conditions at room temperature, the lattice parameter of the crystalline phase LaNi2.18 increases as previously, whereas that of CeNi2.16 is found to decrease with increasing deuterium content. A mechanism for the segregation of the cerium atoms from CeNi2.16 leading to a compound rich in cerium vacancies and cerium hydride is proposed on the basis of the results of the high temperature (90 and 120 °C) hydrogen absorption experiments. The thermal desorption of the amorphous hydride CeNi2.16H3.65 (studied by neutron diffraction) leads to a recrystallization of the intermetallic compound CeNi2 + y after the formation of intermediate CeNix phases.

Thermodynamic, structural and magnetic properties of LaNi5 − xFex hydrides☆

Abstract Thermodynamic, structural and magnetic properties of LaNi 5 − x Fe x hydrides were investigated. The hydrogen absorption and desorption isotherms were determined in the temperature range 25–60 °C using an incremental volumetric method. The substitution of iron for nickel induced a decrease of the plateau pressure and of the hydrogen content. Neutron diffraction measurements showed that the deuteride LaNi 4 -FeD 5.1 has the same structure as the intermetallic compound, the deuterium atoms being distributed over three interstitial sites (6m, 12n and 4h). Hydrogen absorption in LaNi 5 − x Fe x led to a reduction in the magnetization. The Mossbauer spectra of the hydride LaNi 4 FeH 5.3 above the magnetic ordering temperature consisted of a superposition of two quadrupole doublets. The isomer shift was higher than that of the corresponding intermetallic compound.

Hydrogen mobility in LaNi5 hydride and its aluminium-and manganese-substituted hydrides

Abstract Quasi-elastic and inelastic neutron scattering measurements were performed at different resolutions on LaNi 5 and LaNi 4 M (M  Al, Mn) hydrides. The existence of two different types of hydrogen motion in both LaNi 5 and LaNi 4 Mn hydrides is demonstrated: a rapid and fairly localized mode is superimposed on the long-range diffusion mode. Aluminium substitution, which is known to decrease the long-range diffusion coefficient, also induces a strong reduction in the intensity of the local diffusion mode. This rather fast motion can be described using a jump model for the very short distance (0.6 A) between the neighbouring 3f ( z = 0) and 12n ( z ≈ 0.1) sites. This model leads to an incoherent neutron scattering law in good agreement with the measured spectra and to a mean residence time on the 12n site of (1.4 ± 0.2) × 10 −11 s in LaNi 5 H 5.8 and (1.9 ± 0.3) × 10 −11 s in LaNi 4 MnH 5.7 . An inelastic peak centred around 60 meV in LaNi 5 H 5.7 has previously been interpreted as a hydrogen vibrational mode on an octahedral site. We show here that this very broad peak becomes even broader and is strongly reduced in LaNi 4 AlH 4.3 . We suggest that the very close 3f (octahedral) and 12n hydrogen sites cannot be separated in discussions of this optical mode. These quasi-elastic and inelastic studies reinforce the validity of our previous crystallographic analysis.

Study of the different types of hydrogen motion in the β-LaNi5 and β-LaNi4.5Al0.5 hydrides by quasi-elastic neutron scattering☆

Abstract Quasi-elastic neutron scattering measurements were performed on the β-LaNi5 and β-LaNi4.5Al0.5 hydrides using two different energy resolutions. We show that high resolution spectra are composed of at least three components: one elastic line and two lorentzian components. The broader component is attributed to a rapid local motion, and it is studied using a lower resolution as a function of temperature for the aluminium-substituted hydride. Three jump models are discussed for this motion. The dynamic behaviour of hydrogen in both hydrides is well described in the framework of a structural model in which the five available interstices are grouped into two sets of sites, one concentrated around the z = 1 2 plane and the other consisting of clusters of 3f and 12n sites near the basal plane. We show that there are two types of hydrogen atoms: one type exhibits a diffusive motion on the sites of the first set, and the other performs rapid localized jumps within the 3f–12n site clusters and, within the time scale of our experiment, does not appear to diffuse from the clusters towards the z = 1 2 plane. The total scattering law corresponding to this description is given and is compared with our high resolution data. The activation energy for the rapid local motion in the β-LaNi4.5Al0.5 hydride is 170 ± 5 meV.

Structure of Al, Cu and Si Substituted LaNi5 and of the Corresponding β-Deuterides from Powder Neutron Diffraction. Localized Diffusion Mode of Hydrogen in LaNi5 and Al and Mn Substituted Compounds from Quasielastic Neutron Scattering

The first part of this paper gives structural parameters obtained from powder neutron diffraction for the following compounds and related deuterides: LaNi5−xMx with Mx = A10.5, Si0.5, Cu1.0. The second part of the paper is a preliminary report on a neutron time of flight [TOF] spectroscopic measurement which shows for the first time experimental evidence for quasi elastic broadening of hydrogen diffusion peak, both in LaNi5H5.8 and LaNi4MnH5.7. In the A1 substituted phase LaNi4AlH4.3 this effect is much reduced.