Fig. 1. Ball-and-stick representations of (A) the unit cell of the cubic single perovskite CsPb^IIBr₃ and (B) the conventional (four formula units) face-centered-cubic (fcc) cell of the double perovskite Cs₂Ag^IBi^IIIBr₆. (C) Depiction of the interpenetrating B/B′ fcc lattices present in rock-salt-ordered double perovskites such as Cs₂AgBiBr₆. (D) The rhombohedral (one formula unit) unit cell of Cs₂AgBiBr₆. Turquoise, orange, silver, green, and brown spheres represent Pb, Bi, Ag, Cs, and Br atoms, respectively.