Darstellung, Charakterisierung und magnetische Eigenschaften von Übergangsmetallboriden des Th 7 Fe 3 -Typs

摘要

In this work the synthesis, characterization and magnetic properties of new ternary and quaternary transition metal borides with Th7Fe3-type structure were reported. The new compounds were synthesized by arc­melting the elements in a water­cooled copper crucible under argon. In the first part of this work powder samples and single crystals of the ternary boride phases MxRh7-xB3 and MRu6B3 (M = Cr, Mn, Fe, Co, Ni) have been investigated. In the MxRh7-xB3 series single phases were obtained for FeRh6B3, CoRh6B3 und NiRh6B3. The new phases were characterized using single-crystal and powder X-ray diffraction, as well as semi-quantitative EDX measurements. The obtained phases are isotopic and crystallize in the hexagonal Th7Fe3-type structure (space group P63/mc, no. 186, Z = 2). In all cases, M is found to preferentially mix with rhodium at only one of the three available rhodium positions. Paramagnetism was observed in CrxRh7-xB3 and NiRh6B3, whereas ferromagnetic interactions were observed in FeRh6B3 und CoRh6B3. In order to gain more insight into the magnetic ordering observed in these two phases, theoretical calculations of the DFT type were applied. These calculations correctly reproduce not only the lattice parameters but also the ground state magnetic ordering in the two phases. These calculations also show that the long-range magnetic ordering in both phases occurs via indirect ferromagnetic coupling between the iron atoms mediated by rhodium. These two phases are the first ferromagnetic borides adopting the Th7Fe3-type structure. The investigation of the MRu6B3 series (M = Cr, Mn, Co, Ni) showed that the achievement of the ideal composition is problematic. In contrast to the MxRh7-xB3 series the M substitution takes place at two of the three available ruthenium positions. In the second part of this work new series of quaternary borides MRh6-nRunB3 (M = Cr, Fe, Co, Ni; n = 0-5) were synthesized and investigated. The new phases also crystallize in the hexagonal Th7Fe3-type structure and therefore represent the first and well characterized quaternary phases in this family of borides. Single-crystal X-ray diffraction was coupled with EDX analysis to elucidate the atomic arrangement in the crystal structures. M is found in two of the available three metal sites in all cases. An unexpected Ru/Rh site preference is, however, observed in the Ru-rich region. Magnetic properties of the compounds were investigated and for CrRh6-nRunB3 and NiRh6-nRunB3 paramagnetism was observed, whereas for CoRh6-nRunB3 as well as FeRh5RuB3, FeRh4Ru2B3 (Rh-rich phases) and FeRh3Ru3B3, ferromagnetic interactions were observed. The magnetic behavior observed in the Ru-rich phases can be assimilated to the presence of ferro- or ferrimagnetic interactions. In FeRh6-nRunB3 und CoRh6-nRunB3 series the magnetic moment increases with increasing Rh-content according to the Vegard´s rule.