Structure and superconductivity in Zr-stabilized, nonstoichiometric molybdenum diboride

The structure and physical properties of the Zr-stabilized, nonstoichiometric molybdenum diboride superconductor are reported. Good quality material of the diboride structure type can only be obtained by partial substitution of Zr for Mo, and quenching of melts. The diboride phase is best made with boron in excess of the ideal 2:1 boron to metal ratio. Powder neutron diffraction measurements show that the nonstoichiometry is accommodated by atom deficiency in the metal layers. The diboride structure type exists for (Mo_0.96Zr_0.04)_xB₂ for 1.0≥x≥0.85. Electron diffraction shows that the stoichiometric material, x=1, has a significant number of stacking faults. T_c increases from 5.9 to 8.2 K with the introduction of metal vacancies. Resistivity measurements indicate that (Mo_0.96Zr_0.04)_0.88B₂ is a bad metal, and specific heat measurements show its electronic density of states (DOS) is γ=4.4 mJ/molK², and that ΔC/γT_c=1.19. Preliminary boron isotope effect measurements indicate an exponent α=0.11±0.05. Analysis of the data in terms of the electronic structure is reported, allowing an estimate of the electron-phonon coupling constant, λ≈0.1-0.3, making these weak-coupling superconductors. Preliminary characterizations of the superconductivity in the related phases Nb_xB₂ and (Mo_0.96X_0.04)_0.85B₂ for X=Ti and Hf are reported.