Jahn−Teller Distortion-Stabilized Halide Double Perovskites with Unusual Rock-Salt-type Ordering of Divalent B-Site Cations

Halide double perovskites A2B(I)B(III)X6, where the monovalent B(I) and trivalent B(III) cations adopt a rock-salt-ordered arrangement primarily due to their large charge difference and ionic radius difference, have attracted substantial interest in the field of optoelectronics. On the other hand, the combinations of two divalent B-site cations usually result in AB(II)′0.5B(II)″0.5X3 perovskite solid solutions due to the zero charge difference, rather than the rock-salt-ordered A2B(II)′B(II)″X6 double perovskites. In this work, we report the theoretical design and experimental verification of A2B(II)′B(II)″X6 halide double perovskites with unusually rock-salt-ordered divalent B-site cations. Through theoretical calculations, we demonstrate that introducing divalent transition metal cations with partially filled d orbitals into the double perovskite structure can produce significant Jahn–Teller distortions and thus stabilize the rock-salt-ordered double perovskite structure. Among the considered compounds, Cs2HgCuCl6, Cs2HgPdCl6, and Cs2HgPtCl6 were predicted to be thermodynamically stable. Experimentally, Cs2HgCuCl6 and Cs2HgPdCl6 were successfully synthesized as representative examples of the A2B(II)′B(II)″X6 halide double perovskites. The synthesized pure-phase Cs2HgCuCl6 exhibits a small bandgap of 1.08 eV that is suitable for visible light absorption. Our work provides an approach for designing novel double perovskites with two kinds of divalent cations, which expands the large family of halide double perovskites for optoelectronic applications.