  We are reporting a new synthetic strategy to grow large size black phosphorus
(Black-P) crystals through a ternary clathrate Sn$_{24}$P$_{22-x}$I$_8$, under
lower synthetic temperature and pressure. The Black-P crystals are found grown
in situ at the site where the solid clathrate originally resides, which
suggests chemical vapor mineralizer does not play a critical role for the
Black-P formation. More detailed systematical studies has indicated the P
vacancies in the framework of ternary clathrate Sn$_{24}$P$_{22-x}$I$_8$ is
important for the subsequent Black-P from phosphorus vapors, and a likely
Vapor-Solid-Solid (VSS) model is responsible for the Black-P crystal growth.
The obtained room temperature mobility {\mu} is ~ 350 $cm^2/Vs$ from Hall
measurements at mechanically-cleaved flake, where noticeable micro-cracks are
visible. The obtained high mobility value further suggest the high quality of
the Black-P crystals synthesized through this route.
