Synthesis, crystal structure and properties of the new superconductors TaRuB and NbOsB

Two new ternary compounds TaRuB and NbOsB were synthesized by arc-melting and annealing at 1500-1850 degrees C. They crystallize in orthorhombic primitive structures with space group Pbam. Magnetic susceptibility, electrical resistivity, and specific heat measurements reveal bulk superconductivity for metallic TaRuB with a T-c approximate to 4 K. Electronic structure calculations by DFT methods show that 4d and 5d transition-metal states dominate the density of states (DOS) at the Fermi level E-F with a pronounced quasi one-dimensional behaviour along the [0 0 1] direction. Comparison of the calculated DOS at E-F with specific heat data reveals a moderate electron-phonon coupling. Possible small boron vacancies could significantly reduce the DOS at E-F, hence decrease T-c for samples annealed at higher temperatures. For NbOsB, the DOS(E-F) is strongly reduced due to an increase of covalent bonding interactions between Os and B. Accordingly, a lower T-c approximate to 1 K is observed.