Nonahalogenidoarsenates(III) A 3 As 2 X 9 ( A = Rb, Cs, CH 3 NH 3 ; X = Cl─I) as Vacancy‐Ordered Triple Perovskites

The nonahalogenidoarsenates(III) A 3 As 2 X 9 ( A = Rb, Cs, CH 3 NH 3 ; X = Cl─I) were synthesized by reacting As 2 O 3 with the corresponding halide AX in the respective aqueous hydrohalic acids. A 3 As 2 X 9 ( A = Rb, Cs; X = Cl, Br) are crystallized in the Cs 3 Bi 2 Br 9 ‐type structure ( P m 1). The structure is made up of heavily distorted fac ‐[As X 6 ] octahedra, which share three corners each and thus form corrugated 2D layers. A 3 As 2 I 9 ( A = Cs, MA) are crystallizing in the Cs 3 Cr 2 Cl 9 ‐type ( P 6 3 / mmc ). Here, two of the heavily distorted fac ‐ [As X 6 ] octahedra share a common face, forming 0D [As 2 X 9 ] dimers. Such dimers are also observed in Rb 3 As 2 I 9 , but here the LT‐Cs 3 Bi 2 I 9 ‐type ( C 2/ c ) is adopted. MA 3 As 2 Br 9 crystallizes in the Cs 3 Bi 2 Cl 9 ‐type ( Pnma ). Here, four‐membered rings of corner‐linked octahedra form zigzag chains. No compound formation was observed for MA 3 As 2 Cl 9 . In addition to crystal chemistry, the electronic structures and optical properties are discussed.