X-RAY-DIFFRACTION AND EXTENDED X-RAY-ABSORPTION FINE-STRUCTURE STUDY OF RMN(2) HYDRIDES (R=Y, GD OR DY)

Structural properties of RMn(2) hydrides (R = Y, Gd or Dy) have been s tudied by means of powder X-ray diffraction (XRD) at 300 K and extende d X-ray absorption fine structure (EXAFS) at 300 K and low temperature s. It was found that at 300 K the lattice parameters increase continuo usly upon hydrogen absorption (except for DyMn2Hx with x<1) and the cu bic crystal structure is preserved up to x = 3.5 for all three systems . For the fully charged RMn(2)H(4.3), the same type of rhombohedral di stortion of the crystal lattice was found (space group, R(3) over barm $. 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 f ound that except for DyMn2Hx with x < 2 the values of Mn-Mn and Mn-R i nteratomic distances at low temperature agree well with those obtained from XRD measurements and that the hydrides with x = 1 and 2 are loca lly highly disordered. To stress similarities between the three RMn(2) H(x) systems a common schematic phase diagram was constructed. The met hod of preparation of single-phase hydrides was found.