P. Vajda

Low temperature thermal expansion in SmH2+x

Abstract We have measured the lattice parameters in the system SmH1.99+x for 0 ⩽ x ⩽ 0.6 for the first time between liquid nitrogen and room temperature and determined the existence region for the pure β-phase between 1.99(1) and 2.45(3) at H/at Sm. Anomalies in the thermal expansion coefficients are observed near 230 and 190 K, respectively for x = 0.26 and x = 0.45, which are tentatively attributed to ordering of hydrogen atoms in the octahedral sublattice.

On the problem of displacement threshold determination in irradiated metals: subthreshold effects and recovery spectrum

A method for determining the true threshold energy for creating Frenkel pairs in an irradiated metal is proposed. In this method, occasional subthreshold phenomena are separated from the direct displacement process by investigating the stage I annealing spectrum after bombardment with particles having energies close to the suspected threshold.

Study of structural ordering, kondo effects and magnetism in CeH2+x, with 0⩽ x⩽0.65, through electrical resistivity measurements

Abstract Through electrical resistivity measurements on CeH 2+ x , we observe first, for x ⩾0.1, a structural order-disorder transition related to the excess hydrogen content x , which enters the octahedral interstitial sites. For x ⩽ 0.34, we observe Kondo effects and correlated magnetic transitions at low temperatures, which are very sensitive to the hydrogen ordering. For x ⩾ 0.4, we observe a very strong increase of all resistivity terms, i.e. residual, magnetic and phonon contributions: it indicates that there is a change of the electronic structure involving a strong decrease of the conduction electron density; but it could also be due to a percolation problem with the coexistence of metallic and nearly insulating phases.

Observation of the optical and acoustic electron-phonon coupling in Sc, Y and Lu dihydrides and dideuterides by electrical resistivity

The electrical resistivities of Sc, Y and Lu and of their dihydrides and dideuterides have been measured in the temperature range 4.5-300K. They reveal, for the first time in these systems, a contribution from optical phonons which the authors analyse in the harmonic approximation. The coupling to these optical H(D) phonons is found to be much weaker than the corresponding coupling to the acoustic or host phonons. This latter coupling is also weaker in the hydrides than in the hydrogen-free metals. All the observed features are in reasonable agreement with the known facts about the electronic structure of the metals. The absence of superconductivity in the hydrides is also understood in this scheme.

Hydrogen relaxation in yttrium

Abstract The internal-friction spectra of the solid solutions α-YH(D)x with x = 0.05 to 0.2 have been measured between liquid nitrogen and room temperature. They exhibit an hydrogen-related isotope-dependent peak at Tp(H) = 219 K and Tp(D) = 223 K and a break in the dynamic-modulus dependence at the same temperature. The results are discussed in the framework of a Zener-type process of stress-induced rearrangement between H and vacant tetrahedral sites in the ordered low-temperature configuration as established by neutron scattering. The relaxation characteristics of α-YH(D)x are compared with those measured earlier on α-ScH(D)x and α-LuH(D)x and the observed trends related to various lattice properties of the host metal.

The magnetic and structural ordering in the GdH2 + x system

Abstract The excess hydrogen x located on octahedral interstices of the fluorite unit cell in β-GdH2 + x has a tendency to order below room temperature in short-range and long-range ordered structures. We have investigated this ordering by resistivity studies and we propose a phase diagram for the interval 0 ⩽ x ⩽ 0.25. In addition, GdH2 + x displays striking magnetic features, developing from a classic antiferromagnet below TN = 19 K for x = 0 to incommensurate structures for 0

Low-temperature defect study of α-LuH(D)x solid solutions

Abstract Solid solutions of hydrogen or deuterium in lutetium exhibit, when quenched across the well-known 170K anomaly and/or irradiated with electrons at low temperatures, a residual-resistivity increase, Δρq and Δρirr. The former is a linear function of x for LuH(D)x with 0≤x≤0·19. Δρirr manifests a strong isotope effect and varies faster than linearly with x for LuHx, it decreases with increasing electron energy E in the measured interval 0·4≤E≤0·8 MeV (subthreshold with regard to the Lu atoms). The energy dependence of Δρirr has been fitted with electron-hydrogen displacement cross-sections, taking into account the secondary electrons produced by the primaries in the samples. The H(D)-displacement threshold has been assimilated to a binding energy, E b in a short-range ordered configuration; it has been determined from a comparison of the H-resistivities, ρH(T), above and below the anomaly: E b(H) = 63±2 meV and E b(D) = 73±5 meV. The fit of the calculated cross-sections to the experimental data yiel...

Point-defect production in the h.c.p. heavy-rare-earth metals Gd, Tb, Dy, Ho, Er, Tm and Lu during electron irradiation at low temperatures

Abstract Defect production in the h.c.p. heavy lanthanides Gd, Tb, Dy, Ho, Er, Tm and Lu has been investigated by electron irradiation in the energy range 0.7–1.7 MeV, at liquid-helium temperatures. Fitting with displacement cross-sections yielded minimum threshold energies for displacement, T d1 = 14.0 to 17.3 eV, which exhibit the same dependence on the atomic number Z as the reduced reciprocal interatomic distance (c/a)l1.633r 1. A correlation with the sizes of the interatomic ‘lenses’ via the replacement-collision mechanism is suggested. The electrical resistivity increase due to a Frenkel pair is shown to depend both on the phonon resistivity and on the magnetic resistivity of the metal, in the form ρ F ∼ 270ρ ph + 100ρmag at room temperature, implying the introduction of additional spin disorder around a defect.

Electron radiation damage in the rare-earth metals yttrium, samarium and ytterbium at low temperatures

The rare-earth metals Y, Sm and Yb have been irradiated with electrons over the energy range 0.4 to 1.7 MeV at a temperature of Tirr<or approximately=12K, and the damage production as well as the annealing spectra were determined. By fitting the electrical resistivity change rates to theoretical displacement cross sections, the authors have determined the minimum threshold energies for atomic displacement: TdY=(14.6+or-1) eV, Td1Sm=(9.5+or-1.5) eV, Td1Yb=(8.7+or-0.7) eV. Tentative values for the Frenkel pair (FP) resistivities have been proposed taking into account the magnetic order of Sm: rho FY=(50+or-20)*10-4 Omega cm (FP)-1, rho FSm=(140+or-30)*10-4 Omega cm (FP)-1, rho FYb=(75+or-25)*10-4 Omega cm (FP)-1. The threshold energies and the temperatures for interstitial long-range migration for the three metals have been compared with empirical expressions deduced from earlier work and their dependence on the corresponding crystallographic structure and on the ionic configuration has been emphasised.

Defect annealing in the hexagonal close packed heavy rare earth metals Gd, Tb, Dy, Ho, Er, Tm and Lu after electron irradiation at low temperatures

Abstract The recovery spectra of the heavy rare earths Gd, Tb, Dy, Ho, Er and Tm were determined after irradiation with electrons at low temperatures. In the cases of Er and Tm, strong thermal cycling phenomena were observed and attributed to hydrogen atoms changing configurations and interacting with the magnetic structure of the metal. After elimination of these effects in Er and Tm, all the spectra—together with those of the earlier treated Lu—had rather similar form: several close-pair peaks followed by a broad complex substage resembling the stage I D in f.c.c metals and attributed to long-range interstitial migration. A correlation is found between the temperature of the maximum of this substage (normalized to the melting temperature) and the c/a-ratio of the metals.