Structural Polytypism in B-Site-Deficient Azetidinium-Based Pnictogen Halide Hexagonal Perovskites.

Azetidinium (Az+)-based antimony and bismuth organic-inorganic hybrid halide B-site-deficient perovskite analogues Az3B2X9 (B3+ = Sb, Bi; X- = Cl, Br, I) were systematically studied. All Az3B2X9 stoichiometries adopt hexagonal close-packed perovskite structures with the 6H (hcc)2 stacking sequence, differing only in the positions of the ordered B-site vacancies. In Az3Sb2Cl9 and Az3Sb2Br9 ordering of B-site vacancies in a single face-sharing octahedral layer leads to the formation of an unusual 2D layered polar structure with the P63mc space group. Variable-temperature single-crystal and powder XRD, DSC, DTA, and dielectric spectroscopy showed several successive phase transitions at low temperatures associated with distortions of the octahedral framework and order/disorder of the Az+ cation. In contrast, in Az3Sb2I9, Az3Bi2Br9, and Az3Bi2I9, the preferred arrangement of vacancies at corner-sharing octahedral sites generates a 0D "dimer" structure. This work highlights the flexibility for structural variations based on particular configurations of vacancy ordering in B-site-deficient halide perovskites.