  $\gamma$-Graphyne is the most symmetric sp2/sp1 allotrope of carbon, which
can be viewed as graphene uniformly expanded through insertion of two-carbon
acetylenic units between all the aromatic rings. To date, synthesis of bulk
$\gamma$-graphyne has remained a challenge. We here report the synthesis of
multilayer $\gamma$-graphyne through crystallization-assisted irreversible
cross-coupling polymerization. Comprehensive characterization of this new
carbon phase is described, including synchrotron X-ray diffraction, electron
diffraction, lateral force microscopy, Raman and infrared spectroscopy, and
cyclic voltammetry. Experiments indicate that $\gamma$-graphyne is a 0.48 eV
bandgap semiconductor, with a hexagonal a-axis spacing of 6.88 {\AA} and an
interlayer spacing of 3.48 {\AA}, which is consistent with theoretical
predictions. The observed crystal structure has an aperiodic sheet stacking.
The material is thermally stable up to 240 $^\circ$C but undergoes a
transformation at higher temperatures. While conventional 2D polymerizations
and reticular chemistry rely on error correction through reversibility, we
demonstrate that a periodic covalent lattice can be synthesized under purely
kinetic control. The reported methodology is scalable and inspires extension to
other allotropes of the graphyne family.
