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.