Thermal expansion of YMn2Hx compounds

Abstract The thermal expansion of YMn 2 H 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.