Abstract
Molybdenum diboride is unique among transition metal diborides because it exists in both hexagonal (AlB 2 -type) and rhombohedral structures. However, it is difficult to stabilize the superconducting AlB 2 -type phase, which requires either extreme synthesis condition or suitable chemical doping. Here we report the structural and physical properties of Sc-doped nonstoichiometric molybdenum diborides (Mo 0.95 Sc 0.05 ) 1- x B 2 and (Mo 1- y Sc y ) 0.71 B 2 prepared by the common arc melting method. The AlB 2 -type phase is found to form over wide ranges of 0 ≤ x ≤ 0.29 and 0.025 ≤ y ≤ 0.30 for the first time, and bulk superconductivity with T c up to 7.9 K is observed. T c increases with increasing x in the (Mo 0.95 Sc 0.05 ) 1- x B 2 series, but evolves nonmonotonically with varying y in the (Mo 1- y Sc y ) 0.71 B 2 series. Despite this contrast, T c of both borides follows nearly the same linear dependence on the electron-phonon coupling constant, suggesting that it is mainly controlled by the electron-phonon interaction. In addition, the stabilization of AlB 2 -type structure is attributed to the decrease in the number of d electrons as a consequence of Sc doping, which suggests that a similar effect may be achieved by substituting Mo with other d electron-poorer metal elements.
Published Version
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