Electronic, structural and magnetic studies of niobium borides of group 8 transition metals, Nb2MB2 (M=Fe, Ru, Os) from first principles calculations

The Nb2FeB2 phase (U3Si2-type, space group P4/mbm, no. 127) is known for almost 50 years, but until now its magnetic properties have not been investigated. While the synthesis of Nb2OsB2 (space group P4/mnc, no. 128, a twofold superstructure of U3Si2-type) with distorted Nb-layers and Os2-dumbbells was recently achieved, "Nb2RuB2" is still not synthesized and its crystal structure is yet to be revealed. Our first principles density functional theory (DFT) calculations have confirmed not only the experimental structures of Nb2FeB2 and Nb2OsB2, but also predict "Nb2RuB2" to crystalize with the Nb2OsB2 structure type. According to chemical bonding analysis, the homoatomic B–B interactions are optimized and very strong, but relatively strong heteroatomic M–B, B–Nb and M–Nb bonds (M=Fe, Ru, Os) are also found. These interactions, which together build a three-dimensional network, are mainly responsible for the structural stability of these ternary borides. The density-of-states at the Fermi level predicts metallic behavior, as expected, from metal-rich borides. Analysis of possible magnetic structures concluded preferred antiferromagnetic ordering for Nb2FeB2, originating from ferromagnetic interactions within iron chains and antiferromagnetic exchange interactions between them. -- Graphical abstract: Nb2FeB2 (U3Si2 structure type, space group P4/mbm, no. 127) is predicted to order antiferromagnetically, due to the presence of iron chains which show ferromagnetic interactions in the chains and antiferromagnetic interactions between them. "Nb2RuB2" is predicted to crystallize with the recently discovered Nb2OsB2 twofold superstructure (space group P4/mnc, no. 128) of U3Si2 structure type. The building of ruthenium dumbbells instead of chains along [001] is found to be responsible for the stabilization of this superstructure. Highlights: • Nb2FeB2 is predicted to order antiferromagnetically. • Ferromagnetic interactions found in iron chains and antiferromagnetic ones between them. • Unknown "Nb2RuB2" predicted to crystallize with a twofold U3Si2 superstructure. • Puckering of Nb-layer and Ru-dumbbell formation responsible for superstructure occurrence