Single-phase polycrystalline samples and single crystals of the complex boride phases Ti 8 Fe 3 Ru 18 B 8 and Ti 7 Fe 4 Ru 18 B 8 have been synthesized by arc melting the elements. The phases were characterized by powder and single-crystal X-ray diffraction as well as energy-dispersive X-ray analysis. They are new substitutional variants of the Zn 11 Rh 18 B 8 structure type, space group P4/mbm (no. 127). The particularity of their crystal structure lies in the simultaneous presence of dumbbells which form ladders of magnetically active iron atoms along the [001] direction and two additional mixed iron/titanium chains occupying Wyckoff sites 4h and 2b. The ladder substructure is ca. 3.0 Å from the two chains at the 4h, which creates the sequence chain-ladder-chain, establishing a new structural and magnetic motif, the scaffold. The other chain (at 2b) is separated by at least 6.5 Å from this scaffold. According to magnetization measurements, ABSTRACT:Single-phase polycrystalline samples and single crystals of the complex boride phases Ti 8 Fe 3 Ru 18 B 8 and Ti 7 Fe 4 Ru 18 B 8 have been synthesized by arc melting the elements. The phases were characterized by powder and single-crystal X-ray diffraction as well as energy-dispersive X-ray analysis. They are new substitutional variants of the Zn 11 Rh 18 B 8 structure type, space group P4/mbm (no. 127). The particularity of their crystal structure lies in the simultaneous presence of dumbbells which form ladders of magnetically active iron atoms along the [001] direction and two additional mixed iron/titanium chains occupying Wyckoff sites 4h and 2b. The ladder substructure is ca. 3.0 Å from the two chains at the 4h, which creates the sequence chainÀladderÀchain, establishing a new structural and magnetic motif, the scaffold. The other chain (at 2b) is separated by at least 6.5 Å from this scaffold. According to magnetization measurements, Ti 8 Fe 3 Ru 18 B 8 and Ti 7 Fe 4 Ru 18 B 8 order ferrimagnetically below 210 and 220 K, respectively, with the latter having much higher magnetic moments than the former. However, the magnetic moment observed for Ti 8 Fe 3 Ru 18 B 8 is unexpectedly smaller than the recently reported Ti 9 Fe 2 Ru 18 B 8 ferromagnet. The variation of the magnetic moments observed in these new phases can be adequately understood by assuming a ferrimagnetic ordering involving the three different iron sites. Furthermore, the recorded hysteresis loops indicate a semihard magnetic behavior for the two phases. The highest H c value (28.6 kA/m), measured for Ti 7 Fe 4 Ru 18 B 8 , lies just at the border of those of hard magnetic materials.

The electronic structures of "Ti 9-n Fe 2+n Ru 18 B 8 " (n = 0, 0.5, 1, 2, 3), in connection to the recently synthesized Ti 9-n Fe 2+n Ru 18 B 8 (n = 1, 2), have been investigated and analyzed using LSDA tight-binding calculations to elucidate the distribution of Fe and Ti, to determine the maximum Fe content, and to explore possible magnetic structures to interpret experimental magnetization results. Through a combination of calculations on specific models and using the rigid band approximation, which is validated by the DOS curves for "Ti 9-n Fe 2+n Ru 18 B 8 " (n = 0, 0.5, 1, 2, 3), mixing of Fe and Ti is anticipated at both the 2b-and 4h-chain sites. The model "Ti 8.5 Fe 2.5 Ru 18 B 8 " (n = 0.5) revealed that both Brewer-type Ti−Ru interactions as well as ligand field splitting of the Fe 3d orbitals regulated the observed valence electron counts between 220 and 228 electrons/formula unit. Finally, models of magnetic structures were created using "Ti 6 Fe 5 Ru 18 B 8 " (n = 3). A rigid band analysis of the LSDA DOS curves concluded preferred ferromagnetic ordering at low Fe content (n ≤ 0.75) and ferrimagnetic ordering at higher Fe content (n > 0.75). Ferrimagnetism arises from antiferromagnetic exchange coupling in the scaffold of Fe1-ladder and 4h-chain sites. Disciplines Materials Chemistry | Other Chemistry | Physical Chemistry CommentsReprinted (adapted) with permission from J. Am. Chem. Soc., 2011, 133 (17) In the past two decades a class of complex intermetallic borides has been synthesized containing magnetically active 3d atoms in close proximity to each other, allowing for studies of magnetic exchange as a function of valence electron count. 1À4 Some of these compounds are variants of the Zn 11 Rh 18 B 8 -type structure, which crystallizes in the P4/mbm (no. 127) space group. Substitution of zinc by both titanium and iron, along with replacing rhodium with ruthenium, leads to the previously reported compound Ti 9 Fe 2 Ru 18 B 8 . 5 This structure contains 'ladders' of iron atoms where the 'rungs' are formed by Fedimers with an interatomic distance of ca. 2.5 Å and separated by ca. 3.0 Å along the [001] direction. The distances are short enough for through-space magnetic exchange to occur; as a result, the magnetic properties of this compound were investigated both experimentally and theoretically. 5 Ti 9 Fe 2 Ru 18 B 8 was determined to order ferromagnetically with a magnetic moment of 1.2 μ B at 7 T and a Curie temperature (T C ) of 200 K. The Weiss constant (θ) is approximately þ290 K, further indicating a strong (FeÀFe) ferromagnetic exchange interaction. 5 The magnetic ordering was also predicted to be ferromagnetic by theory. An analysis of the crystal orbital Hamilton populations (ÀCOHP) and the density of states (DOS) curves showed the occupation of FeÀFe antibonding states and a local maximum in the nonmagnetic DOS at the Fermi level, both of which point toward electronic instability in the system. 6,7 Allowing the structure to relax through spin polarization resulted in the removal...

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Powder samples and single crystals of the Ti

Transition-metal borides show not only promising physical properties but also a rich variety of crystal structures. In this context, quantum-chemical tools can shed light on important facets of the chemistry within such intermetallic borides. Using density-functional theory (DFT), we analyze in detail two phases of significant structural-chemical importance: the recently synthesized Ti(1+x)Os(2-x)RuB(2) and the isotypical Ti(1+x)Os(3-x)B(2). Starting from the observation of different Ti/Os occupations in X-ray crystal structure analysis, we assess suitable computational models and rationalize how the interplay of Ti-Ti, Ti-Os, and Os-Os bonds drives the site preferences. Then, we move on to a systematic investigation of the metal-boron bonds which embed the characteristic, trigonal-planar B(4) units within their metallic surroundings. Remarkably, the different Ti-B bonds in Ti(1+x)Os(2-x)RuB(2) (and also in its ternary derivative) are of vastly different strength, and the strength of these bonds does not correlate with their length. The tools presented in this work are based on simple and insightful chemical arguments together with DFT, and may subsequently be transferred to other intermetallic phases--transition-metal borides and beyond.