Structural and bonding characteristics of potassium-doped p-terphenyl superconductors

Recently, there is a series of reports by Wang et al. on the superconductivity in K-doped p-terphenyl (K_xC₁₈H₁₄) with the transition temperatures ranging from 7 to 123 K. Identifying the structural and bonding characteristics is the key to understand the superconducting phases and the related properties. Therefore, we carried out an extensive study on the crystal structures with different doping levels and investigated the thermodynamic stability, structural, electronic, and magnetic properties by the first-principles calculations. Our calculated structures capture most features of the experimentally observed X-ray diffraction patterns. The K-doping concentration is constrained to within the range of 2-3. The obtained formation energy indicates that the system at x = 2.5 is more stable. The strong ionic bonding interaction is found in between K atoms and organic molecules. The charge transfer accounts for the metallic feature of the doped materials. For a small amount of charge transferred, the tilting force between the two successive benzenes drives the system to stabilize at the antiferromagnetic ground state, whereas the system exhibits nonmagnetic behavior with increasing charge transfer. The multiformity of band structures near the Fermi level indicates that the driving force for superconductivity is complicated.