  The potassium-doped p-terphenyl compounds were synthesized in recent
experiments and the 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 the first-principles
simulation. We first find out the low energy position of K atom in intralayer
interstitial space of the molecular layer, then examine whether two rows of K
atoms can be accommodated in this one space, at last the effect of the
interlayer arrangement between adjacent two molecular layers on total energy is
taken into account. Our results show that the doped K atoms prefer to stay at
the bridge site of single C-C bond connected 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 Kx-p-terphenyl, the
K2-p-terphenyl phase with P212121 group symmetry is determined to be most
appropriate, which is different from the one in 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.
