THERMAL-EXPANSION OF YMN2HX COMPOUNDS

The thermal expansion of YMn2Hx compounds (x=0.5, 2, 3) has been studi ed by means of low temperature X-ray powder diffraction. Both the magn etic ordering temperature and the lattice parameter a of the cubic Lav es phase increase with increasing x. The volume anomalies at the magne tic 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 su ppressed with increasing x, are responsible for an enhanced thermal ex pansion in the paramagnetic range. Very interesting results were obser ved 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 possib le ordering of the hydrogen atoms within the cubic C15-Laves phase str ucture.