P. Vajda’s research while affiliated with French National Centre for Scientific Research and other places

After a brief review of the structural and electronic properties of the non-stoichiometric rare-earth hydrides, RH2+x, we discuss metal-semiconductor transitions observed in several β-phase RH2+x-systems in the temperature range 200 to 300 K for x-values close to the phase boundary to the trihydride γ-phase. The special case of YH2+x is treated in detail, showing the evolution from a purely metallic case with x=0.085 to a clear M-S transition for x=0.10 (at the β-phase limit), passing through a fluctuating situation for x=0.095. The essential role played by an order — disorder transformation occurring in the octahedral (x) hydrogen sublattice as the driving mechanism for the M-S transition is emphasized. A second M-S transition observed at low temperatures (≲ 80 K) is attributed to weak carrier localisation due to x-hydrogen disorder.

Results from first-principles investigations of the energetical, structural, electronic, and vibrational properties of model structures probing the metal-rich region of the lanthanum-hydrogen system, i.e., the region of the solid solution of hydrogen in lanthanum, are presented. We have studied the site preference and the ordering tendency of hydrogen atoms interstitially bonded in close-packed lanthanum. Spatially separated hydrogen atoms have turned out to exhibit an energetical preference for the occupation of octahedral interstitial sites at low temperature. Indications for a reversal of the site preference in favor of the occupation of tetrahedral interstitial sites at elevated temperature have been found. Linear arrangements consisting of pairs of octahedrally and/or tetrahedrally coordinated hydrogen atoms collinearly bonded to a central lanthanum atom have turned out to be energetically favorable structure elements. Further stabilization is achieved if such hydrogen pairs are in turn linked together so that extended chains of La-H bonds are formed. Pair formation and chain linking counteract the energetical preference for octahedral coordination observed for separated hydrogen atoms.

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Results from first-principles investigations of the structural, electronic, and vibrational properties for three concentration ranges (stoichiometries close to elemental La, LaH2, and LaH3) within the La–H system are presented.

Both one dimensional H-2 MAS NMR and two dimensional (EXSY) NMR were used to study lanthanum sub-stoichiometric trideuteride (LaD2.85) at room temperature. Remarkably narrow (8 and 10 Hz) intense lines were observed at 3.06 and 2.97 ppm in addition to two additional (31 and 2 35 Hz wide) at 2.62 and 1.92 ppm. The shifts were measured with respect to the H NMR line in PMMA (poly(methylmethacrylate)). The positions and intensities of the lines are discussed in terms of the order-disorder situation in the octahedral D-sublattice of the lanthanum matrix.

Sodium fluoride crystals were bombarded with MeV electrons near room-temperature or at 21K and their EPR spectra measured. While after RT-irradiation one observes the spectrum of the classical cubic F-center, the low-T irradiation introduces a new low-symmetry defect which transforms reversibly into the F-center above ≈200K. We suggest for the new center a triplet VK-type defect stabilized during irradiation by the nearby created F-center. While the ordinary VK-center as observed after low-T X-ray and γ-irradiation vanishes at 200K the present electron-induced defect is stable and reappears upon cooling; it is still discernible after anneals up to 200°C.

Comment to a paper by Ito et al. concerning a relation between lattice parameter and electronegativity in metal hydrides. It is shown that this relation cannot be applied for (Ti,Zr)H2 on one hand and YH2 on the other, the claimed “correlation” being due to erroneous interpretation of experimental data for the latter.

Electrical resistivity measurements on CeH2+x specimens show and incoherent and a coherent Kondo lattice behaviour above and below T ≈ 20 K, respectively. The incoherent term has a log T variation showing the influence of the crystal-field degeneracies. We observe also low-temperature anomalies related to magnetic transitions which tend to disappear when atomic disorder is introduced by quench into the sublattice of the supplementary (x) H-atoms. The coefficient of the low-temperature (T 2 K) T2-law of ρ(T) is in good agreement with the heavy-fermion model.

We have irradiated Li2NH powder with MeV electrons at room temperature and investigated the introduced defects with electron spin resonance. Conduction electron spin resonance indicates the presence of nanosize metallic Li colloids seen as a Lorentzian line with a g=2.0023 and a linewidth DeltaH=50muT . A second, broader signal (DeltaH=3-4mT) appears superimposed upon the Li line at low T (Curie-type behavior) which exhibits complex T dependence with a break near 180K for its g value and DeltaH . We are suggesting for the latter a vacancy-type defect in the NH sublattice, with freezing of its H component below 180K . When heated both the Li colloids and the color centers anneal around 100°C probably due to hydrogen evolution and subsequent chemical degradation.

We are reviewing recent progress obtained in the study of hydrogen interaction with magnetic structures in rare-earth metals, emphasizing the role of the ordering state of the H-sublattice. We shall present in particular: (1) the situation in the solid solutions of the heavy rare earths, α∗-RHx, and the efforts to relate the special quasi-unidimensional configuration of this phase (charge-density waves) to their modulated magnetism (spin-density waves) via the Fermi surface; (2) an assembly of all magnetic structures determined up to now in the heavy-RE dihydrides, β-RH(D)2(+x), for a tentative comparison of the observed commensurate and/or incommensurate configurations; special mention will be made of the remarkably stable short-range ordered (SRO) magnetism always present in these systems; (3) the need for further (thorough) neutron scattering work on the magnetism in the insulating R-trihydrides, γ-RH(D)3(−x), in order to specify its mechanism in the absence of RKKY exchange interaction.