Synthesis, crystal structure and hydrogenation properties of rhenium-substituted YMn_2 and ErMn_2

摘要

Hexagonal C14-type Laves phases have been synthesized by sintering for the Y(Mn_(1-x)Re_x)_2 and Er(Mn_(1-x)Re_x)_2 series in the concentration intervals 0.5 < = x < 1 and 0 < x < 1, respectively. These were examined with respect to hydrogenation at temperatures of up to 473 K and pressures of up to 200 bar. Absorption plateaus occur only at low rhenium contents and very low hydrogen pressures (Y system: approx 0.2 bar at 473 K for x = 0.6, Er system: ~0.05 bar at 433 K for x = 0.2). The maximum H contents achieved at 1 bar and 473 K are 1.4 H/f.u. and 3.4 H/f.u. in the yttrium (x = 0.6) and erbium (x = 0.2) series, respec-tively. As the rhenium content increases the absorption capacity decreases for both series at a similar rate. The reactions appear to be reversible. In contrast to the rhenium-free end members no reconstruc-tion of the metal substructure occurs. X-ray and synchrotron diffraction suggest that rhenium substitutes manganese randomly on two atomic sites for which the Re/Mn ratios do not change significantly during hydrogenation. Neutron diffraction at various deuterium contents suggests that tetrahedral A_2B_2-type interstices are filled first (4 sites) and then AB_3-type interstices (1 site) (A = Y or Er; B = Mn+Re). The relative occupancy of these sites depends on different factors and is compared with other C14-type deuterides.