  Based on first-principles calculations, we predict Bi2TeI, a stoichiometric
compound synthesized, to be a weak topological insulator (TI) in layered
subvalent bismuth telluroiodides. Within a bulk energy gap of 80 meV, two
Dirac-cone-like topological surface states exist on the side surface
perpendicular to BiTeI layer plane. These Dirac cones are relatively isotropic
due to the strong inter-layer coupling, distinguished from those of previously
reported weak TI candidates. Moreover, with chemically stable cladding layers,
the BiTeI-Bi2-BiTeI sandwiched structure is a robust quantum spin Hall system,
which can be obtained by simply cleaving the bulk Bi2TeI.
