Stabilization and superconductivity of AlB2-type nonstoichiometric molybdenum diboride by Sc doping

Molybdenum diboride is unique among transition metal diborides because it exists in both hexagonal (AlB2-type) and rhombohedral structures. However, it is difficult to stabilize the superconducting AlB2-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 (Mo0.95Sc0.05)(1-x)B-2 and (Mo1yScy)(0.71)B-2 prepared by the common arc melting method. The AlB2-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 (Mo0.95Sc0.05)(1-x)B-2 series, but evolves nonmonotonically with varying y in the (Mo1-yScy)(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 electronphonon interaction. In addition, the stabilization of AlB2-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.

Automated summary

This study reports the structural and physical properties of a molybdenum diboride material prepared by arc melting method. The material is found to exhibit superconductivity in the AlB2-type phase, and the superconducting transition temperature is influenced by the electron-phonon interaction.