  Lead-based halide perovskites are emerging as the most promising class of
materials for next generation optoelectronics. However, despite the enormous
success of lead-halide perovskite solar cells, the issues of stability and
toxicity are yet to be resolved. Here we report on the computational design and
the experimental synthesis of a new family of Pb-free inorganic halide
double-perovskites based on bismuth or antimony and noble metals. Using
first-principles calculations we show that this hitherto unknown family of
perovskites exhibits very promising optoelectronic properties, such as tunable
band gaps in the visible range and low carrier effective masses. Furthermore,
we successfully synthesize the double perovskite Cs2BiAgCl6, we perform
structural refinement using single-crystal X-ray diffraction, and we
characterize its optical properties via optical absorption and
photoluminescence measurements. This new perovskite belongs to the Fm-3m space
group, and consists of BiCl6 and AgCl6 octahedra alternating in a rock-salt
face-centered cubic structure. From UV-Vis and PL measurements we obtain an
indirect gap of 2.2 eV. The new compound is very stable under ambient
conditions.
