J.P. Burger

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.

An analysis of the electrical conductivity of the two phase Pd Hx system

Abstract The electrical conductivity of the two phase region of the Pd Hx (0 ≤ x ≤ 0.65) system at 20K is described by a new semi-phenomenological effective medium type equation. This equation has been shown to describe the conductivity of a wide variety of binary mixtues as a function of the conductivities of the two components (ϱα and ϱβ in the case of Pd Hx) and two morphology parameters. One parameter is the critical volume fraction (ƒ′ c (= 1 − ƒ c ) at which the high conductivity phase first forms a continuous percolation path across the system. The other parameter, the exponent t, is a combination of the effective demagnetization coefficients of the components and the critical volume fraction. This parameter is discussed in terms of the known morphology of the samples.

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.

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

Correlations between magnetism, electrical conductivity and structural ordering in rare earth hydrides REH2+x

The excess hydrogen concentration x has fundamental effects on the magnetic properties which are due to modification of the conduction electron density and the shape of the Fermi surface, to crystal field effects and to order-disorder transitions within the H sublattice. We discuss: i) the occurrence of commensurate (for x = 0) and non-commensurate (for x > 0) antiferromagnetic ordering; ii) the effects of the crystal field which give rise to a non-Kondo resistivity minimum for PrH 2 ; iii) the incoherent and coherent Kondo effects of CeH 2+x and its relation with magnetism and structural ordering.

Magnetic properties of NdH2 + x compounds☆

We report magnetization measurements for NdH2 + x compounds (x = 0 to 0.75) as a function of temperature (1.5 K < T < 15 K) and field (H < 15 kOe). For the stoichiometric composition NdH2 the system behaves like a conventional ferromagnet with strong coercivity effects. However, as x increases it gradually evolves towards non-ferromagnetic or spin-glass-like behaviour. This is reflected both in the temperature and field dependence of the magnetization. Our data are consistent with the idea that for x ≠ 0 there is no strict long-range ferromagnetic ordering, because it is suppressed by the so-called random uniaxial anisotropies which could be induced by the crystal field and the local fluctuations in the hydrogen concentration. This effect occurs when the ratio of the random anisotropy energy to the exchange energy approaches unity, a condition which is probably fulfilled in the neodymium hydrides.

Optical and acoustic electron-phonon coupling in Ho and Tb dihydrides and dideuterides

Abstract We present detailed measurements of the electrical resistivity for terbium and holmium dihydrides and dideuterides. An analysis of the thermal variation, ϱ(T), permits to separate two contributions, one stemming from the acoustic or host metal ion vibration and a second which is related to the optical H (or D) vibrations and which is strongly isotope dependent. The thermal variations can be reasonably well represented by a Debye spectrum as concerns the acoustic phonons and by an Einstein spectrum as concerns the optical phonons. We find also that the strength of the electron-phonon coupling is much weaker for the optical phonons as compared to the acoustic ones, in agreement with recent estimates from the theory.

ESR in the Pd-Mn system: Some evidences of a bottlenecked relaxation for the Mn2+ ion☆

Abstract ESR experiments on Pd-Mn in the low Mn concentration region (200 ⩽ c ⩽ 5000 at. ppm) are reported. The results are consistent with the hypothesis of a bottlenecked resonance with different g-factors for the Mn2+ ions (2.055) and the electrons of Pd (2.25); besides, partial opening of this bottleneck can be achieved. A giant momentum for Mn2+ (9.15μB) is also confirmed.

E.S.R. of Mn2+ and Gd3+ ions in palladium and in the palladium-hydrogen system

Abstract Observation of g -shifts and line-widths in Pd and PdH indicate the occurrence of a bottleneck for Mn 2+ ions, probably due to s -electrons, for both matrices. Hydrogen ions do not seem to act as relaxation centres for the conduction electrons towards the lattice. No clear evidence for bottlenecks has been found until now for Gd 3+ ions. The sign of the g -shift is reversed when we go from Pd to PdH: this puts the sign of Δg in line with current theories for the PdH matrix which is probably very near to being an s -type metal. The signs for Δg in pure Pd are not yet well understood.