W. Iwasieczko

Hydrogenation equilibria characteristics of LaNi5−xZnx intermetallics

Abstract A series of metal hydride alloys were prepared based on the LaNi 5 formula, by substituting 0, 5, 10, 15 and 20 at.% of Zn for Ni. The obtained pseudobinary LaNi 5− x Zn x alloys have been characterized by X-ray diffraction patterns and hydrogen pressure–composition–temperature ( PCT ) diagrams at temperatures ranging from 293 to 353 K. The P – C isotherms were used to determine thermodynamic functions (enthalpy and entropy) for the hydrogen desorption process. Increasing Zn substitution causes the unit cell volume to increase and the plateau pressures to decrease. The maximum concentration of hydrogen in the tested La(Ni,Zn) 5 alloys slightly decreases with increasing Zn content.

Hydrogen absorption properties of ZrNi5−xCox alloys

Abstract The structural and hydrogen sorption properties of the ZrNi 5− x Co x ( x =0,1,2 and 3) system have been studied. The alloys were characterized by X-ray diffraction patterns and by pressure-composition-temperature (PCT) diagrams at temperatures ranging from 448 to 523 K. It was found that compositions with 0≤ x ≤2 were solid solutions of AuBe 5 type structure, whereas the ZrNi 2 Co 3 alloy showed a two-phase structure. Substitution of Ni by Co increases the unit cell volume of the monophase alloys by 0.84% per one substituted atom in formula unit. Hydrogen uptake increases the cell volume of the host alloy by up to 2.3%. In the studied conditions of temperature (448 to 523 K) and pressure (0.05 to 2.5 MPa) the PCT diagrams do not exhibit any plateau of hydrogen equilibrium pressure. The hydrogen capacity increases with increasing Co substitution. The largest hydrogen solubility has been found for the ZrNi 2 Co 3 alloy (1.3 hydrogen atoms per formula unit).

Low-temperature specific heat of non-stoichiometric β-ytterbium deuteride

Heat capacity measurements on a β-ytterbium deuteride sample with the composition D/Yb=2.71 have been carried out over a temperature range from 1.9 to 260 K. Two anomalies were observed at 4 K and 230 K, respectively. On the grounds of the separated Schottky contribution to the specific heat, the influence of the crystal field on the splitting of the (2J+1)-fold degenerate ground state J=7/2 of the Yb3+ ion is analysed. The existence of two sublattices (sort of an electronic phase separation) of hydrogen atoms arranged by the two different ytterbium atom valencies is suggested. The sharp discontinuity in the specific heat at 230 K in YbD2.71 resembles that observed in the light rare earth hydrides REHx in the composition range 2.7<x<2.92 and attributed to a metal-semiconductor transition. X-ray diffraction measurements in the range 50–300 K indicate that the anomaly in Cp(T) at 230 K is accompanied by the probable tetragonal distortion of the cubic structure at low temperatures. The metal-insulator/semiconductor transformation is likely induced when the deuterium/hydrogen atoms order in the structure.

Magnetic properties of the ytterbium non-stoichiometric trihydride phase

Abstract Magnetic susceptibility measurements of non-stoichiometric ytterbium trihydrides have been performed. A transition to the antiferromagnetic state at temperatures just below 4.2 K has been found. The effective magnetic moment calculated per ytterbium ion increases with hydrogen content from 3.18 μB for H Yb =2.30 to 3.97 μ B for frso|H/Yb = 2.62, respectively. These changes are connected with the increase of the amount of trivalent ytterbium ions with hydrogen increasing concentration.

Magnetic properties of the β- and β′-ytterbium hydride phases

Abstract Samples of the β-ytterbium hydride phase with compositions 2.2 B for various compositions across the β-ytterbium hydride phase. This effective moment is smaller than the ionic Hund’s rule moment of 4.54 μ B expected for trivalent Yb 3+ ions and clearly indicates that the Yb atoms stay in an intermediate valence state. The average valence of Yb ions ν (Yb)=2.72±0.02 calculated from magnetic data is independent on the hydrogen concentration and lie between the expected values for the corresponding pure valent Yb 2+ and Yb 3+ ions. For the first time, the magnetic properties of the β′-ytterbium hydride phase are also measured and discussed as well. The obtained results indicate that the ytterbium atoms in the cubic β′-hydride phase are trivalent like the light lanthanide elements in their hydride phases.

The magnetocaloric effect and magnetic transitions in hydride compounds: GdNiH3.2 and TbNiH3.4

The paper presents the investigation of GdNiH3.2 and TbNiH3.4 hydrides magnetic transitions and magnetocaloric properties. The isothermal magnetization data in the fields up to 5T are obtained for GdNi and TbNi compounds and their hydrides and the values of magnetic entropy change are calculated. The maximum values of magnetic entropy change ΔSM in GdNiH3.2 and TbNiH3.4 are extremely large. It is shown that the hydrogenation shifts ΔSM(T) maximum to lower temperatures.