Niobium substitution suppresses the superconducting critical temperature of pressurized MoB₂

A recent study has demonstrated that MoB2, transforming to the same structure as MgB2 (P6/mmm), superconducts at temperatures above 30 K near 100 GPa [C. Pei et al., Natl. Sci. Rev. 10, nwad034 (2023)], and Nb substitution inMoB(2) stabilizes the P6/mmm structure down to ambient pressure [A. C. Hire et al., Phys. Rev. B 106, 174515 (2022)]. The current work explores the high-pressure superconducting behavior of Nb-substituted MoB2 (Nb0.25Mo0.75B2). High-pressure x-ray diffraction measurements show that the sample remains in the ambient pressure P6/mmm structure to at least 160 GPa. Electrical resistivity measurements demonstrate that from an ambient pressure T-c of 8 K (confirmed by specific heat to be a bulk effect), the critical temperature is suppressed to 4 K at 50 GPa, before gradually rising to 5.5 K at 170 GPa. The critical temperature at high pressure is thus significantly lower than that found in MoB2 under pressure (30 K), revealing that Nb substitution results in a strong suppression of the superconducting critical temperature. Our calculations indeed find a reduced electron-phonon coupling in Nb0.25Mo0.75B2, but do not account fully for the observed suppression, which may also arise from inhomogeneity and enhanced spin fluctuations.