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 AzBX (B = Sb, Bi; X = Cl, Br, I) were systematically studied. All AzBX stoichiometries adopt hexagonal close-packed perovskite structures with the 6H (hcc) stacking sequence, differing only in the positions of the ordered B-site vacancies. In AzSbCl and AzSbBr 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 6 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 AzSbI, AzBiBr, and AzBiI, 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.