Stable Structural Phase of Potassium-Doped p-Terphenyl and Its Semiconducting State

Potassium-doped p -terphenyl compounds were synthesized in recent experiments and superconductivity with high transition temperatures were reported, but the atomic structure of potassium-doped p -terphenyl is unclear. In this paper, we studied the structural and electronic properties of potassium-doped p -terphenyl with various doping levels by first-principles simulation. We first found out the low-energy position of K atom in the intralayer interstitial space of the molecular layer, then examined whether two rows of K atoms can be accommodated in this single space, and at last the effect of the interlayer arrangement between two adjacent molecular layers on the total energy was taken into account. Our results show that the doped K atoms prefer to stay at the bridge site of the single C–C bond connecting two phenyls instead of locating at the site above the phenyl ring, distinct from the situation of K-doped picene and phenanthrene. Among the possible structural phases of K x - p -terphenyl, the K 2 - p -terphenyl phase with P 2 1 2 1 2 1 group symmetry is determined to be the most appropriate, which is different from the one in a recent report. The stable K 2 - p -terphenyl phase is semiconducting with an energy gap of 0.3 eV, and the bands from the lowest unoccupied molecular orbitals are just fully filled by the electrons transferred from K atoms.