Ti3Co5B2 (space group P4/mbm, no. 127) is a prolific structure type within the metal-rich borides family, which has produced more than 60 isotypic phases. The structure of the ternary (A3T5B2 or Ti3Co5B2) can be described as face connected trigonal, tetragonal and pentagonal prisms of Co or T atom building channels. Ti or A atom occupies the tetragonal or pentagonal prism whereas boron resides in the trigonal prism. In the quaternaries (A2MT5B2), the magnetically active elements M resides inside the tetragonal prism and forms chains in the c direction, with intrachain M-M distance of ca. 3.0 Å. Such quaternary compounds show interesting magnetic properties, for example, antiferromagnetism (Sc2FeRu5B2), ferromagnetism (Sc2FeRh5B2), metamagnetism (Sc2FeIr5B2). The variation in magnetic ordering in these compounds was understood to depend on the number of valence electrons (VE) in the light of theoretical calculations. Following that, many phases have been synthesized where the magnetic ordering was varied by just changing the number of valence electrons while keeping the magnetically active element unchanged. Within the series Sc2FeRu5–nRhnB2 (n =0−5, VE = 60−65 e-), gradually increasing the number of VE enabled to tune the magnetic ordering from antiferromagnetic to ferromagnetic. Interestingly, using the 5d element iridium led to the series Sc2FeRu5–xIrB2 (n =0−5, VE = 60−65 e-), two members (with 62 and 63 e-) of which possess the highest coercivity measured for transition metal borides so far, up to 52.4 kA/m. These recent results motivated us to search for new rare-earth-free magnets crystallizing in the Ti3Co5B2 structure type. For this purpose, we presumed that inclusion of heavier 5d transition elements (Zr, Hf) along with 3d magnetic elements should lead to higher magnetic moments in those compounds, as hinted by previous observations and theoretical studies. So far, no boride containing Hf/Zr together with magnetically active elements crystallizing in the Ti3Co5B2 is reported. Only (Hf/Zr)3Ru5B2 and HfIr5B2 were synthesized recently Our synthetic efforts started with gradual replacement of Ru by magnetically active elements (Fe, Co, Ni) in the known ternary Hf3Ru5B2. We have successfully replaced Ru by M (Fe, Co, Ni) in Hf3Ru5B2 via arc melting. Refinements of the X-ray diffraction data show that the desired phases were the major product with other known byproducts. M was present both at Hf (Wyckoff site 2a) and Ru (Wyckoff site 2c) positions.

Synthesis and characterization of Hf3Ru5-xMxB2 (M = Fe, Co, Ni) –towards rare-earth-free magnets

253rd National Meeting (2017)

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