Covalent bond inducing strong electron-phonon coupling superconductivity in MgB2-type transition metal diborideWB(2)

A recent experiment of polycrystalline WB2 with hP3 (space-group 191, prototype MgB2) and hP12 (space-group 194, prototype WB2) structures was reported to realize 17-K superconductivity (SC) at 90 GPa, and the hP3 structure is believed to be responsible for this emergent SC. However, a microscopic understanding of what makes the hP3 structure so different from the hP12 structure and why the hP3 can feature such strong electron-phonon coupling (EPC) SC is still missing. Here, based on first-principles calculations, we found that in the hP3 structure, W d orbitals contribute most to electronic occupation near the E-F, and d(z)(2) orbitals of two neighboring W atoms have some hybridization to form weak sigma bonds. The further EPC analysis indicates that the dominant d(z)(2) states are strongly coupled with the out-of-plane phonon modes by stretching theW-W sigma bond, thereby yielding a large superconducting gap and high T-c of similar to 35 K. By contrast, for the hP12 structure, two neighboring W atoms are isolated without charge hybridization to form the covalent bonds, and, accordingly, their phonon modes become very stiffened, which cannot effectively couple to W d orbital states associated with a lower T-c of similar to 4 K. Therefore, our findings not only provide an explanation for the emergent strong EPC SC in the hP3 structure, but also have important implications for the design of high-T-c superconductors among transition metal borides.