  Using first-principles GW plus Bethe-Salpeter equation calculations, we
identify anomalously strong excitonic effects in several vacancy-ordered double
perovskites Cs2MX6 (M = Ti, Zr; X = I, Br). Giant exciton binding energies
about 1 eV are found in these moderate-gap, inorganic bulk semiconductors,
pushing the limit of our understanding of electron-hole (e-h) interaction and
exciton formation in solids. Not only are the exciton binding energies
extremely large compared with any other moderate-gap bulk semiconductors, but
they are also larger than typical 2D semiconductors with comparable
quasiparticle gaps. Our calculated lowest bright exciton energy agrees well
with the experimental optical band gap. The low-energy excitons closely
resemble the Frenkel excitons in molecular crystals, as they are highly
localized in a single [MX6]2- octahedron and extended in the reciprocal space.
The weak dielectric screening effects and the nearly flat frontier electronic
bands, which are derived from the weakly bonded [MX6]2- units, together explain
the significant excitonic effects. Spin-orbit coupling effects play a crucial
role in red-shifting the lowest bright exciton by mixing up spin-singlet and
spin-triplet excitons, while exciton-phonon coupling effects have minor impacts
on the strong exciton binding energies.
