J.-M. Joubert

Anisotropic diffraction peak broadening and dislocation substructure in hydrogen-cycled LaNi5 and substitutional derivatives

An erroneous equation and some consequently underestimated values of dislocation densities in the paper by Cerný et al. [J. Appl. Cryst. (2000), 33, 997–1005] are corrected.

Hydrogen cycling induced degradation in LaNi5-type materials

Defect generation in hydrogen cycled LaNi5 and substituted derivatives was studied in previous work by analysis of the X-ray line broadening. In the present work, the pulverization in the same samples is analyzed by granulometric measurements and scanning electron microscopy. Both phenomena correspond to irreversible degradation of the initial intermetallic compounds, as confirmed by first cycle hysteresis presence in pressure composition isotherms. They are analyzed in terms of the lattice expansion occurring at the discrete phase transition between α and β phases and measured by X-ray diffraction. This study shows that, in addition to this latter parameter, the limit of elasticity and the resistance to rupture must be considered.

Improvement of the electrochemical activity of ZrNiCr Laves phase hydride electrodes by secondary phase precipitation

Abstract Within the framework of research on intermetallic compounds for nickel-hydride batteries, we have studied ternary ZrNiCr alloys. In that system, we have measured both the hydride thermodynamic properties by solid-gas reaction and the electrochemical capacities for ternary Laves phases and binary ZrNi intermetallic compounds. A knowledge of the ternary phase diagram has allowed the synthesis of alloys where these two kinds of phase are in equilibrium, at predetermined rates. Study of the electrochemical capacities of these two-phase alloys has shown that the surface modification induced by the precipitation of ZrNi binaries considerably enhances the electrochemical capacity discharged by the Laves phase.

Hydrogen absorption properties of several intermetallic compounds of the ZrNi system

Abstract This paper deals with the hydrogen absorption properties of four binary compounds of the Zr Ni system: Zr 8.85 Ni 11.15 , Zr 7 Ni 10 , Zr 8 Ni 21 and Zr 2 Ni 7 . Hydrogen capacities, absorption kinetics and pressure-composition isotherms were measured by the volumetric method. We have observed, except for Zr 2 Ni 7 , an increase of both capacity and stability of hydrides as a function of zirconium content in the intermetallic compound.

Crystallographic study of LaNi5−xSnx (0.2≤x≤0.5) compounds and their hydrides

The structural properties of tin substituted LaNi5 compounds and corresponding deuterides have been investigated by single crystal X-ray and neutron powder diffraction. Tin is found to substitute nickel exclusively on site 3g of space group P6/mmm. No other significant disorder occurs in the structure. At tin contents of x=0.4 and x=0.5, the crystal structures of the deuterides do not significantly deviate from those of other metal substituted LaNi5 deuterides. At a tin content of x=0.2, with higher deuterium content, however, a symmetry decrease to non-centrosymmetric space group P6mm is observed due to partial deuterium ordering.

Site Occupancies in the Battery Electrode Material LaNi3.55Mn0.4Al0.3Co0.75 as Determined by Multiwavelength Synchrotron Powder Diffraction

The battery electrode material LaNi 3.55 Mn 0.4 Al 0.3 Co 0.75 is a substitutional derivative of LaNi 5 in which La occupies site 1(a), and Ni, Mn, Al and Co sites 2(c) and 3(g) of space group P6/mmm. The distribution of the substituting elements Mn, Al and Co on the two latter sites was determined from synchrotron powder diffraction experiments by using the diffraction contrast produced by anomalous dispersion effects. The results of joint Rietveld refinement on three patterns collected close to the K edges of Ni, Co and Mn and on one pattern collected away from these edges showed that Al and Mn occupy almost exclusively site 3(g), while Co is distributed over sites 2(c) and 3(g) with a slight preference for site 3(g).

Effects of cobalt replacement by nickel, manganese, aluminium and iron on the crystallographic and electrochemical properties of AB5-type alloys

Abstract Cobalt has been partially or totally replaced by other elements such as Ni, Mn, Al or Fe in MmNi4.07Mn0.63Al0.2Co0.4 composition in order to evaluate the effects on microstructure and electrochemical properties (capacity, cycle life, corrosion…). The cycle life of the electrodes for which cobalt has been replaced by nickel or manganese is strongly diminished. However, interestingly, Fe-containing electrodes behave as well as Co-containing electrodes in terms of cycle life, despite a slight capacity reduction. Moreover, the SEM picture revealed that in a similar way to Co, Fe reduces the alloy pulverisation during cycling. This result is in agreement with a low discrete lattice expansion for these Fe-containing alloys.

Contribution of the Rietveld method to non-stoichiometric phase modeling. Part II: α-Tl5Te3 and μ Nb-Ni as experimental examples

Two detailed examples are considered to show the ability of powder diffraction data analyzed with the Rietveld method to get insight into the detailed structure of non-stoichiometric phases. Both the γ-phase in Tl-Te system and μ-phase in Nb-Ni system were investigated as a function of composition across their whole homogeneity domain. In the case of the γ Tl-Te phase, the excess Tl relative to the stoichiometric composition is accommodated by Tl substitution on one of the two Te sites. In the μ Nb-Ni, three of the five sites of the structure are partially disordered for all the compositions studied. The results are analyzed in view of Calphad modeling of both phases.

χ and σ phases in binary rhenium-transition metal systems: a systematic first-principles investigation.

The Frank-Kasper phases, known as topologically close-packed (tcp) phases, are interesting examples of intermetallic compounds able to accommodate large homogeneity ranges by atom mixing on different sites. Among them, the χ and σ phases present two competing complex crystallographic structures, the stability of which is driven by both geometric and electronic factors. Rhenium (Re) is the element forming the largest number of binary χ and σ phases. Its central position among the transition metals in the periodic table plays an important role in the element ordering in tcp phases. Indeed, it has been shown that Re shows an opposite site preference depending on which elements it is alloyed with. In the present work, χ- and σ-phase stability in binary Re-X systems is systematically studied by a first-principles investigation. The heats of formation of the complete set of ordered configurations (16 for χ and 32 for σ) have been calculated in 16 well-chosen systems to identify stability criteria. They include not only the systems in which χ-Re-X (X = Ti, Mn, Zr, Nb, Mo, Hf, Ta, W) or σ-Re-X (X = V, Cr, Mn, Fe, Nb, Mo, Ta, W) exist but also the systems in which both phases are not stable, including systems in which X is a 3d element from Ti to Ni, a 4d element from Zr to Ru, and a 5d element from Hf to Os. Careful analysis is done of the energetic tendencies as a function of recomposition, size effect, and electron concentration. Moreover, the site preference and other crystallographic properties are discussed. Conclusions are drawn concerning the relative stability of the two phases in comparison with the available experimental knowledge on the systems.

Thermodynamic and structural comparison between two potential metal-hydride battery materials LaNi3.55Mn0.4Al0.3Co0.75 and CeNi3.55Mn0.4Al0.3Co0.75

Abstract The thermodynamic and structural properties of CeNi3.55Mn0.4Al0.3Co0.75, LaNi3.55Mn0.4Al0.3Co0.75 and their hydrides (deuterides) are compared. The isothermal curves have been measured. The lattice parameters and the occupancies of the various interstitial sites by deuterium have been determined by powder neutron diffraction. Differences between the two compounds as regards the hydrogen absorption are interpreted in terms of the different atomic radii of lanthanum and cerium. The evolution of the valence of cerium as a function of hydrogen content is discussed.