Tuning superconductivity in high entropy nitrides

The recently investigated nitrogen (N) incorporation into TiNbMoTaW high entropy alloy films leads to a domelike superconducting transition temperature 𝑇𝐶 vs nitrogen concentration 𝑥 (with 𝑥=N/𝑀 representing the ratio between atomic fractions of nitrogen and metal atoms, 𝑀=Ti+Nb+Mo+Ta+W) in the respective high entropy nitride (TiNbMoTaW)1.0⁢N𝑥, with a large 𝑇𝐶 enhancement. In this study, we report on the effect of N insertion into NbTaHfTiZr films, which show a lower valence electron concentration and a lower configuration entropy than those of TiNbMoTaW. The 𝑇𝐶 vs 𝑥 dependence in (NbTaHfTiZr)1.0⁢N𝑥 exhibits a different course, in which 𝑇𝐶 first decreases from ∼6.5K at 𝑥=0 to 𝑇𝐶≈1K at 𝑥=0.25 and 𝑥=0.42, and then starts to increase and reaches a maximum of ∼8.6K at 𝑥=0.81 followed by a decline to ∼6.7K when 𝑥 approaches the stoichiometric value 1. In this case, the transition of the initial high entropy alloy bcc structure to the fcc structure of high entropy nitrides proceeds slowly and leads to a wide 𝑥 range in which 𝑇𝐶 is suppressed. The analysis shows that the 𝑇𝐶 vs 𝑥 course and the 𝑇𝐶 enhancement in high entropy nitrides depend on the valence electron concentration of the initial high entropy alloy, as higher values accelerate the tendency to form a fcc structure and are less sensitive to the localization of otherwise mobile electrons in metallic high entropy alloys due to their bonding to N atoms. However, other parameters, such as the atomic size difference of the metallic constituents and their affinity towards nitrogen, also have to be considered.