First-principles study of structural, electronic and superconducting properties of XB (X = Mo, Zr)

Transition metal borides have attracted significant attention in the exploration of superconducting materials due to their unique structural and electronic properties. Based on first-principles calculations, this study systematically reveals the significant differences in superconducting properties between MoB and ZrB, along with their underlying microscopic mechanisms. The calculations predict that MoB exhibits a superconducting transition temperature ([Formula: see text] of 6.96[Formula: see text]K at ambient pressure, which is significantly higher than that of its isostructural counterparts ZrB (1.61[Formula: see text]K) and WB (2.79[Formula: see text]K). Its superconductivity primarily originates from the strong coupling between Mo-4d electrons and low-frequency optical phonons. Importantly, studies on pressure effects show that the [Formula: see text] of MoB decreases monotonically with increasing pressure, dropping to 2.12[Formula: see text]K at 150[Formula: see text]GPa. Analysis of the microscopic mechanism indicates that this suppression is directly related to the reduction in the density of states at the Fermi level and the hardening of key phonon modes. Electronic structure analysis further reveals that the electronic states at the Fermi surface are predominantly contributed by transition-metal d-orbitals (accounting for [Formula: see text]85%). Furthermore, crystal orbital Hamiltonian population (COHP) analysis reveals that the superior mechanical strength of MoB (bulk modulus: 346.81[Formula: see text]GPa) originates from its strongest Mo–B bonding. This study clarifies the central role of transition-metal d-electron states in regulating the superconducting properties of such borides and proposes that enhancing the coupling between d-electrons and low-frequency phonons is an effective strategy for designing novel superconducting materials.