Thermal expansion of YMn₂H_x compounds

The thermal expansion of YMn₂H_x compounds (x = 0.5, 2, 3) has been studied by means of low temperature X-ray powder diffraction. Both the magnetic ordering temperature and the lattice parameter a of the cubic Laves phase increase with increasing x. The volume anomalies at the magnetic ordering temperatures decrease with increasing hydrogen content x. This is attributed to the increasing localization of the Mn moments. Spin fluctuations in the paramagnetic state, which are successively suppressed with increasing x, are responsible for an enhanced thermal expansion in the paramagnetic range. Very interesting results were observed for x = 0.5. In this compound there is a huge tetragonal distortion which changes its sign at 105 K. This outstanding behaviour may be due to a highly anisotropic magnetoelastic interaction caused by a possible ordering of the hydrogen atoms within the cubic C15-Laves phase structure.