- Silvie Mašková: Hydrogen absorption in RE2T2X compounds with Mo2FeB2 structure: crystal structure and magnetism
- 27. 4. 2016, 14:50
- lecture room F2, first floor Ke Karlovu 5
- more information
Abstract:
Large number of the ternary f-metal intermetallics with the stoichiometry "221" crystallize in the Mo2FeB2 structure type (space group P4/mbm). This tetragonal structure is interesting from the point of view of the magnetic properties, as the magnetically active f-atoms are arranged as dimers in the basal plane, which opens the possibility for various exotic features of the magnetization, depending on the relation of the exchange coupling parameters within the dimer and between the dimers. Considering the role of the balance between the intra- and inter-dimer exchange, we have decided to use the hydrogenation as the tool for weakening their absolute and relative strengths.There was found that all studied RE2T2X compounds can absorb hydrogen. Several types of reaction of RE2T2X to hydrogen were found. One type is the expansion of the tetragonal unit cell found RE upon absorption of 2 H/f.u. at RE2Pd2In with heavy. Another type is amorphization (found at RE2Pd2In for early RE = La, Ce, Nd), yielding > 4 H/f.u. Orthorhombic distortion (RE2Ni2In, RE = Nd, Pr,Ce, La) can accommodate up to 7 H/f.u. The monoclinic structure found for RE2Ni2Mg hydrides can apparently have even higher H concentration approaching 8 H/f.u. The crystal-structures of all these hydrides can be obtained by distortion of the starting Mo2FeB2 structure. On the other hand, RE2Pd2Mg is representing different reaction of the crystal structure on hydrogenation. Due to the rearrangement of atoms in RE2Pd2MgH8 the structure of the hydride cannot be regarded as simple derivative of the structure of the initial compound.
Magnetic properties of RE2T2X compounds studied are dramatically influenced by hydrogen absorption. Although the hydrides remain antiferromagnetic similarly to the parent compounds, the respective Néel temperatures TN decrease considerably. For RE compounds, hydrogenation affects mainly the 4f – magnetic moments and their ordering. The exchange coupling is reduced presumably by reducing the concentration of conduction electrons.