Rb4CuSb2X12 (X = Cl, Br) as Novel Vacancy-Ordered Quadruple Perovskites

The vacancy-ordered quadruple perovskites Rb<inf>4</inf>CuSb<inf>2</inf>X<inf>12</inf> (X = Cl, Br) were synthesized from acidic solutions and by solid-state reaction. Although both crystal structures are described as new distortion variants of vacancy-ordered quadruple perovskites, the coordination of the Sb atoms differs depending on the halides (X = Br: CN = 2 + 2 + 2; X = Cl: CN = 4 + 1 + 2). The similarities and differences of the two compounds to each other as well as to the perovskite structure are discussed on the basis of group-subgroup relationships. In addition, the optical properties (bandgap of approx. 0.63/0.82 eV, Br/Cl) and the electronic structure of Rb<inf>4</inf>CuSb<inf>2</inf>Br<inf>12</inf> are discussed. This also addresses the difficulties in accurately determining the electronic structure of compounds containing Cu²⁺ ions using DFT-based methods.