X-ray diffraction and extended X-ray absorption fine-structure study of RMn2 hydrides (R = Y, Gd or Dy)

Abstract

Structural properties of RMn2 hydrides (R = Y, Gd or Dy) have been studied by means of powder X-ray diffraction (XRD) at 300 K and extended X-ray absorption fine structure (EXAFS) at 300 K and low temperatures. It was found that at 300 K the lattice parameters increase continuously upon hydrogen absorption (except for DyMn2Hx with x < 1) and the cubic crystal structure is preserved up to x = 3.5 for all three systems. For the fully charged RMn2H4.3, the same type of rhombohedral distortion of the crystal lattice was found (space group, R3m). For the hydrogen concentration range 3.5 < x < 4.3, coexistence of cubic C15 and rhombohedral phases with various contents was found. The local atomic order was studied by means of the EXAFS method. It was found that except for DyMn2Hx with x < 2 the values of Mn-Mn and Mn-R interatomic distances at low temperature agree well with those obtained from XRD measurements and that the hydrides with x = 1 and 2 are locally highly disordered. To stress similarities between the three RMn2Hx systems a common schematic phase diagram was constructed. The method of preparation of single-phase hydrides was found.