via niobium substitution*

Recently, superconductivity was discovered in diborides like MoB₂ and WB₂. It was found that MoB₂, in an MgB₂-like structure, superconducts at temperatures above 30 K near 100 GPa. Following these discoveries, we explored the high-pressure superconducting behavior of Nb-substituted MoB2 (Nb0.25Mo0.75B2). High pressure x-ray diffraction measurements revealed that Nb_0.25Mo_0.75B₂ stays in P6/mmm structure to at least 160 GPa. Resistivity measurements showed a T_c of 8.15 K (confirmed by specific heat to be a bulk effect) at ambient pressure. The T_c is suppressed to ∼ 4 K at 50 GPa, before gradually rising to ∼ 5.5 K at 170 GPa. The critical temperature at high pressure in Nb_0.25Mo_0.75B₂ is significantly lower than that found in MoB₂ at high pressure (30 K), revealing that Nb-substitution results in a strong suppression of the superconducting critical temperature. The calculated Allen Dynes T_c at high pressure is significantly higher than the observed T_c. Possible explanations for these observations are discussed.