Rock-Salt-Ordered Nitrohalide Double Antiperovskites: Theoretical Design and Experimental Verification

Motivated by the success of metal halide perovskites in the field of optoelectronics, antiperovskites, which have the same crystal lattice as that of perovskites but with switched anion and cation positions, have recently been considered as potential candidates for optoelectronic applications. However, the best-known oxyhalide antiperovskites A3OX (A = alkali metal; X = halogen) generally exhibit very large bandgaps due to their immensely strong ionicity. In this work, we report the theoretical design and experimental verification of rock-salt-ordered double antiperovskites with reduced bandgaps. Each of two O anions in the A3OX oxyhalide antiperovskites is replaced with the combination of a trivalent N anion and a monovalent Y″ anion, yielding A6NY″X2 double antiperovskites. Density functional theory (DFT) calculations reveal that compared with the A3OX antiperovskites, the A6NY″X2 double antiperovskites have significantly reduced bandgaps, mainly due to high-lying N 2p orbitals. On the other hand, the p orbitals of the Y″ anion lie far below the N 2p orbitals and almost do not affect the bandgaps. Guided by the DFT calculations, we successfully synthesized four nitrohalide double antiperovskites, that is, Li6NBrBr2, Li6NII2, Li6NClBr2, and Li6NBrI2, and verified the predicted bandgap reductions. Our work provides strategies for designing double antiperovskites and expands the large family of antiperovskites.