Planar B6 rings embedded in one-dimensional Ti7 wheels are found in the crystal structure of a solid-state phase (Ti7Rh4Ir2B8) for the first time. First-principles calculations indicate strong BB bonding but also significant interactions with the surrounding titanium atoms (see picture).
The two phases, MRh6B3 (M = Fe, Co) are isotypic and crystallize in the hexagonal Th7Fe3 structure type (space group P63mc, no. 186, Z = 2). In this structure, the magnetically active atoms (Fe, Co) are preferentially found on only one of the three available rhodium sites, and together with rhodium they build a three-dimensional network of interconnected (Rh/M)3 triangles. Magnetic properties investigations show that both phases order ferromagnetically below Curie temperatures of 240 K (for FeRh6B3) and 150 K (for CoRh6B3). First-principles DFT 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.
Through a combination of calculations on specific models and using the rigid band approximation, which is validated by the DOS curves for “Ti9-nFe2+nRu18B8” (n = 0, 0.5, 1, 2, 3), mixing of Fe and Ti is anticipated at both the 2b- and 4h-chain sites. The model “Ti8.5Fe2.5Ru18B8” (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 “Ti6Fe5Ru18B8” (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.