Structural and magnetic properties of C15 HoMn2 hydrides

Powder samples of cubic HoMn2Hx hydrides, with 0 ≤ x ≤ 4.3, have been investigated by X-ray diffraction and AC/DC magnetometry as a function of temperature and external magnetic field. Hydrogen is demonstrated to strongly modify structural and magnetic properties. X-ray studies revealed many structural transformations placed at low temperatures. In particular, a transformation from the cubic to the monoclinic structure was detected, which so far has not been reported for other cubic RMn2Hx (R – rare earth or Yttrium) compounds. The structural transformations are reflected in the magnetic behavior. The change in ordering temperatures implies a very strong relationship between the magnetic interactions and the Mn–Mn distance modified at hydrogen absorption. Tentative magnetic and structural phase diagrams are proposed. The presented results are compared with the properties of other cubic and hexagonal RMn2Hx hydrides.

Research highlights▶ Full structural and magnetic phase diagrams are presented for the HoMn2Hx(0 < x ≤ 4.3) hydrides, where x = 4.3 is a typical maximal hydrogen concentration obtained for RMn2Hx (R: rare earth) under low pressure of H2 (ca ∼ 1 bar). ▶ For x < ∼ 1.6: (1) The spinodal decompositions into two structurally different phases are observed. The lattice parameters of one of them correspond to those of the pure host HoMn2 material, while the lattice parameters of the other phase correspond to those of HoMn2H1.6. (2) Abundances of both phases change with hydrogen concentration x. (3) No intermediate phase appears, which is typical for other hydrides based on the C15 Laves phase type compounds (e.g. (Tb, Gd)Mn2). Instead of that, structural transformations to monoclinic structures are observed. ▶ For x = 2.5 splitting into two phases with different hydrogen concentrations x (x1 ∼ 2.3 and x2 ∼ 2.8) appears again, which was observed only for hydrides based on the C14 Laves phase type compounds (e.g. in (Sm, Er, Nd)Mn2Hx) in the range 2 < x < 3. ▶ For samples with 2 < x < 2.2 no structural transformations are detected. ▶ The effects of hydrogen absorption on structural properties are shown to be reflected in magnetic behavior. ▶ A huge jump of magnetic ordering temperatures from ∼24 K for HoMn2 to ∼ (200–380) K for its hydrides is observed.