A. V. Kolomiets

Hydrogen absorption–desorption, crystal structure and magnetism in RENiAl intermetallic compounds and their hydrides

Abstract At ambient pressures RENiAl (RE=Y, Gd, Tb, Dy, Er and Lu) ternary intermetallic compounds crystallising in the ZrNiAl type of crystal structure form hydrides containing up to 1.4 H/f.u. Hydrogenation leads to a drastic reduction of magnetic ordering temperatures. It is accompanied by changes of magnetic structures and, for the majority of compounds, by orthorhombic distortion of the initial hexagonal symmetry.

RNiAl hydrides and their magnetic properties

Abstract R NiAl compounds absorb hydrogen at normal pressure typical to a stoichiometry of R NiAlH = 1.4 . Thermal desorption studies showed that, besides such hydrides, several partly decomposed hydrides with lower H concentrations are also stable. The X-ray analysis shows that, in most cases, hydrogenation leads to an orthorhombic distortion of the original hexagonal structure of the ZrNiAl type. The hydrogenation severely affects magnetic properties depressing ordering temperatures and changing all the magnetic phase diagrams.

Magnetic properties of UFeGe

UFeGe crystallizes in the orthorhombic structure of the TiNiSi type with a monoclinic distortion. Magnetic susceptibility and electrical resistivity measurements point to paramagnetic behaviour with characteristic features of spin fluctuations. Electronic structure calculations reveal a strong hybridization of the 5f and 3d states with the Fermi energy located in a minimum of the density of states.

Evidence for an extended critical region near the metamagnetic transition of UCoAl

Electrical resistivity and ac magnetic susceptibility studies of the 5f-metamagnet UCoAl reveal the presence of strong spin fluctuation effects for temperatures below 30 K and magnetic fields below the critical metamagnetic field μ0Hc≈1 T, where a non-Fermi liquid term ∼T3/2 is prominent in the electrical resistivity. A step-wise increase of the residual resistivity and an increasingly prominent Fermi liquid T2 term of the resistivity is observed as the field is increased though the metamagnetic transition.

Hydrogen-induced changes in TbNiAl

Neutron diffraction was used to determine the deuterium sites in TbNiAlD1.28 and TbNiAlD0.8. Both samples were found to crystallize in the orthorhombic Amm2 space group. Our analysis reveals that deuterium in TbNiAlD1.28 occupies positions at x=0 and x=1/2 only, while TbNiAlD0.8 shows the formation of two different phases. For TbNiAlD1.28, additional magnetic reflections are observed below 16 K indicating antiferromagnetic order. On the other hand, at 10 K, only one of the TbNiAlD0.8 phases was found to be magnetic. In addition, our study of the localized vibrations of hydrogen in TbNiAlH1.39 and in its non-magnetic analog YNiAlH1.35 using inelastic neutron scattering supports the existence of two different hydrogen sites in the saturated hydrides.

Structural and magnetic properties of equiatomicrare-earth ternaries

Abstract Hydrogenation was found to be a strong tool for modification of magnetic propertiesand crystal structure of the RE NiAl intermetallic compounds. Non-uniform expansion ofthe unit cell due to incorporation of hydrogen leads to an orthorhombic distortion of the originalZrNiAl-type hexagonal structure. Hydrogenation also largely suppresses magnetic order in allstudied compounds: it weakens exchange interactions, lowers ordering temperatures and modifiesmagnetic phase diagrams. A series of hydrides with lower H concentration was studied, as well,and non-continuous variation of their properties with reducing H content indicates that the maineffect of hydrogenation is not of geometrical character but due to the variation of electronicdensity.

f-Element hydrides: structure and magnetism

Abstract Systematic magnetic and specific-heat studies of hydrides of ternary compounds on the basis of rare earths or actinides help to unravel different roles of f-states in these two classes of materials. In rare earths, H absorption tends to suppress magnetism by weakening the exchange interaction mediated by conduction electrons. Light actinides exhibit a 5f band of variable width at the Fermi level and the hydrogenation makes magnetic features typically stronger. 5f band narrowing due to volume expansion is responsible for this net tendency, but the effects of H bonding cannot be neglected.

Magnetism in hydrogenated UTGe compounds

UTGe compounds form α-hydrides with a small H concentration and β-hydrides accommodating up to 2 H atoms/f.u. Hydrogen absorption has a significant impact on the 5f magnetism. The Curie temperature-TC of suspected ferromagnetic superconductor UCoGe increases to 50 K. Magnetic susceptibility of spin fluctuator UFeGe increases, but long-range magnetic order is not achieved. For ferromagnetic URhGe only α-hydride could be obtained, which moderate increase of TC and spontaneous moment. The α-hydride of UCoGe is surprisingly paramagnetic and non-superconducting despite of a small volume expansion (0.25 %).

Magnetism of UTGe compounds tuned by hydrogen absorption

UTGe compounds absorb hydrogen up to 2 H atoms/f.u., which has a dramatic impact on the 5f magnetism. The TC value of UCoGe increases to 50 K. Magnetic susceptibility of spin fluctuator UFeGe markedly increases. For ferromagnetic URhGe only α-hydride could be obtained, which moderate increase of TC and spontaneous moment. The α-hydride of UCoGe is surprisingly paramagnetic despite of a small volume expansion (0.25 %).

Stability of the non-Fermi liquid state in UCoAl

Abstract A non-Fermi liquid (NFL) power law behaviour of the resistivity with exponent n= 5 3 was found in the low-field state of the band metamagnet UCoAl. This characteristic is rather robust, with weak dependence on magnetic field lower than the critical metamagnetic field. The critical field increases, while the anomaly at the metamagnetic transition is gradually reduced, with applied pressure.