▎ 摘　　要

The thriving area of synthetic carbon allotropes witnesses theoretic proposals and experimental syntheses of many new two-dimensional monolayer structures, which are often achieved by careful arrangement of non-hexagon sp(2) defects in graphene. Here, we introduce pyramid sp a hybridization into sp(2) network and propose a new carbon polymorph with clathrate pattern and minimum egg-tray shape (termed as clathrate graphene). Eight symmetrically equivalent sp(2) carbon atoms and two symmetrically equivalent sp(2) carbon atoms in its tetragonal primitive unit cell form two perpendicularly oriented rectangles and four bridging hexagons. Though deformed bond lengths and bond angles, the planar geometry of all tetrarings and hexagons are retained. High percentage and small deformation of hexagons make this metastable sp(2)-sp(3) allotrope comparable with sp(2) T-graphene and penta-graphene in energetics. Exhaustive ab initio calculations confirm its dynamical and elastic stabilities, reveal its semiconducting nature with an indirect band gap of 0.90 eV in PBE level (1.90 eV in HSE06 level) for unstressed sample. Strain effects on the elastic and electronic properties of this allotrope are also studied, an ultrahigh ideal strength that outmatches graphene and a giant strain tuning effect on its bandgap are observed. These salient properties may imply potential nanoelectronic applications. Our findings help understand structure-property relationship for two-dimensional carbon allotropes, and help search new carbon polymorphs.