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

Area of Science:

• Materials Science
• Solid-State Chemistry
• Crystallography

Background:

• B-site deficient perovskite analogues offer unique structural possibilities.
• Azetidinium (Az⁺) cations introduce organic-inorganic hybrid characteristics.
• Understanding vacancy ordering is key to controlling material properties.

Purpose of the Study:

• To systematically investigate Azetidinium-based antimony and bismuth halide perovskite analogues (Az₃B₂X₉).
• To elucidate the impact of B-site vacancy ordering on crystal structure.
• To explore the resulting structural diversity and phase transitions.

Main Methods:

• Synthesis and characterization of Az₃B₂X₉ compounds (B = Sb, Bi; X = Cl, Br, I).
• Variable-temperature single-crystal and powder X-ray diffraction (XRD).
• Differential Scanning Calorimetry (DSC), Differential Thermal Analysis (DTA), and dielectric spectroscopy.

Main Results:

• All Az₃B₂X₉ compounds adopt hexagonal close-packed perovskite structures (6H stacking).
• Az₃Sb₂Cl₉ and Az₃Sb₂Br₉ form 2D layered polar structures due to vacancy ordering in face-sharing octahedra.
• Az₃Sb₂I₉, Az₃Bi₂Br₉, and Az₃Bi₂I₉ form 0D dimer structures from vacancies in corner-sharing octahedra.
• Low-temperature phase transitions were observed in Az₃Sb₂Cl₉ and Az₃Sb₂Br₉, linked to octahedral distortions and Az⁺ cation disorder.

Conclusions:

• Vacancy ordering in B-site deficient perovskites dictates the formation of distinct structural motifs (2D layered vs. 0D dimer).
• Azetidinium-based halide perovskites demonstrate significant structural flexibility.
• This study provides insights into the structure-property relationships of novel hybrid perovskite materials.