B-Site Columnar-Ordered Halide Double Perovskites A2B(II)′0.5B(II)X5 with B(II)′/Vacancy Disordering

B-site rock-salt-ordered halide double perovskites have attracted significant attention for optoelectronic applications such as light-emitting diodes and photodetectors. Recently, B-site columnar-ordered halide perovskites A2B(I)B(II)X5 have been discovered as a new family of halide double perovskites, providing a new platform for exploring optoelectronic semiconductors. In this work, we report a new class of B-site columnar-ordered halide double perovskites A2B(II)′0.5B(II)X5, which are derived from the B-site columnar-ordered halide double perovskites A2B(I)B(II)X5 by replacing every two B(I) cations by one B(II)′ cation and one vacancy. Three examples, that is, Cs2Hg0.5PdCl5, Cs2Hg0.5PdBr5, and Cs2Hg0.5PtCl5 have been synthesized by a solvothermal method. The synthesized halide double perovskites crystallize into a tetragonal structure with space group P4/mmm, where the two nonequivalent crystallographic B-sites adopt the columnar ordering. The stoichiometric B(II)′ cations and vacancies are disorderly arranged on the equivalent octahedral B-sites. These halide double perovskites exhibit relatively small bandgaps of 1.93–2.05 eV and good air and thermal stability. This work further expands the family of B-site columnar-ordered halide double perovskites and provides guidance for theory-driven material exploration.