J.C. Achard

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.

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.

Hydrogenation of carbon monoxide on carbon-supported cobalt rare earth catalysts

A study of carbon monoxide hydrogenation over cobalt—lanthanum and cobalt—cerium catalysts supported on carbon was carried out for the purpose of identifying the effects of rare earth elements. It was observed that these promoters give rise to a 100-fold increase in specific activity and turnover frequency, which is not a result of the inhibition of carbon monoxide or hydrogen adsorption. Moreover, the selectivity for the C2C4 fraction increased from 4% to about 40% and this fraction is essentially olefinic. Such variations in catalytic properties, especially activity, are a strong indication of the formation of new sites. In comparison with the results of recent work, one may conclude that carbon monoxide and hydrogen hydrocondensation needs redox centres that could easily be formed with metal-rare earth catalysts.

Enthalpies of formation and hydrogenation of La(Ni(1−x)Cox)5 compounds

Abstract The enthalpies of formation of La(Ni(1−x)COx)5 compounds have been measured using a calorimetric method. The variation of these values as a function of composition is discussed using a tight-binding model to describe the electronic structure of these compounds. The enthalpy and entropy of formation of hydrides are deduced from the study of the change of their equilibrium pressures as a function of temperature. Correlations between structural and thermodynamic properties of the intermetallic compounds of the La(Ni1−x)Cox)5 series and their related hydrides are discussed.

Structural characterization of RNi2 (R ≡ La, Ce) intermetallic compounds and their hydrides☆

Abstract It has been determined that the cubic phase LaNi2 (MgCu2 type) exists in a small homogeneity range (2.17 Extended X-ray absorption fine structure (EXAFS) studies at the Ni K edge for RNi2 (R ≡ La, Ce) Laves phases compounds has revealed a local disorder for both compounds which is more pronounced for LaNi2 than for CeNi2. X-ray powder diffraction measurements of the intermetallics on hydrogenation show an increase in the cell parameter for LaNi2 up to LaNi2H1 ( ΔV V = 1.4% ) and a decrease for CeNi2H1 ( ΔV V = − 0.5% ). Beyond 1 H atom mol−1 the cell parameter is almost constant. Above 3.5 H atom mol−1 the X-ray diffraction lines disappear completely. Transmission electron microscopy, magnetization and X-ray absorption spectroscopy (EXAFS and Ce LIIIedge) measurements allow us to conclude that these hydrides are amorphous (for XH⩾ 3.5). In addition, a change in the cerium valency occurs on hydrogenation.

Enthalpies of formation of liquid and solid binary alloys of lead, antimony and bismuth with rare earth elements

Abstract The predictions of Miedemas model for the heats of formation of binary intermetallic compounds of lead, bismuth and antimony with rare earth elements and for the heats of mixing and solution of the corresponding liquid alloy systems are presented. The predicted values are compared with the existing experimental data. The differences between the experimental and the calculated values of the enthalpy are discussed.

Experimental and calculated enthalpies of formation of rare earth-tin alloys

Abstract The partial enthalpies of some rare earth elements (lanthanum, cerium, thulium, lutetium and yttrium) at infinite dilution in liquid tin were determined by means of a calorimetric method. The change in this thermodynamic property with atomic number is discussed in terms of the contribution of the 4f electrons to the bonding. This assumption is used to calculate the hybridization term involved in Miedemas relation. Experimental results therefore allow an accurate determination of the parameters of Miedemas relation. These are used to calculate the enthalpies of formation of rare earth-tin compounds. The predicted values are compared with available experimental data.

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.

Localization of hydrogen (deuterium) in α-LaNi5Hx (x=0.1 and 0.4)

Abstract Powder neutron diffraction studies have been performed for the compounds α-LaNi 5 D x ( x = 0.1 and 0.4). In the P 6/ mmm space group of the intermetallic LaNi 5 , in the case of the “virgin” sample the deuterium atoms are found in the basal plane for the two compounds with strains localized in this plane but the deuterium atoms are displayed in all crystallographic directions for the “activated” sample. A geometrical model applied to these compounds revealed that in the basal plane only the 12n site can be occupied. Our structural determination did not exclude the occupancy of a second site but gave a lower occupancy than the 12n site.