Peierls‐Distorted Ru‐Chains and Boron Dumbbells in Nb2RuB2 and Ta2RuB2 from First‐Principles Calculations and Experiments

Abstract

Nb2RuB2 and Ta2RuB2 phases were recently predicted by GGA‐VASP structure optimization to crystallize in the Nb2OsB2‐type structure. Although the Fe‐based (Mo2FeB2 type) and Os‐based (Nb2OsB2 type, superstructure variant of Mo2FeB2 type) analogues have been synthesized and characterized successfully, the Ru‐based phases remained unknown. Crystal structure prediction of Nb2RuB2 and Ta2RuB2 phases, using an evolutionary algorithm, led to the AlMn2B2‐type structure in contrast to the aforementioned optimization; however, phonon calculations showed that the Nb2OsB2‐type phases are dynamically more stable than the AlMn2B2‐type phases. A slightly modified synthetic strategy finally led to the successful preparation of the predicted phases. The extremely quick arc‐melting procedure, under argon atmosphere, not only led to a quantitative amount of the phases but also to single crystals suitable for structure determination. Powder and single‐crystal X‐ray diffraction as well as EDX analysis of the metal ratio have confirmed the GGA‐VASP structure optimization: Nb2RuB2 and Ta2RuB2 compounds indeed crystallize isotypically with Nb2OsB2 structure, a superstructure variant of Mo2FeB2 type, in which B‐dumbbells and Peierls‐distorted Ru‐chains are found. Susceptibility measurements on a Ta2RuB2 single crystal reveal no superconducting transition down to 2 K, even though some features in the band structures of both phases, similar to those reported in superconducting NbRuB, hinted at possible superconductivity.