Vacancy ordering and polytypism in B-site deficient halide perovskites

Abstract

B-site deficient halide perovskites show promising optoelectronic properties as potential candidates for the replacement for conventional lead-based halide perovskites. To further understand the relationship between crystal structure and properties, it would be of great interest to explore B-site deficient halide perovskites with various vacancy distributions. This work focuses on the preparation and characterisation of novel B-site deficient halide perovskites with unusual vacancy ordering.

The successful preparation of Cs₄CuSb₂Cl₁₂ in 2017 provided insight into the study of cubic closed packed B-site deficient halide perovskites. Theoretical screening and experimental work were carried out in this study to explore other potential A₄B’²⁺B³⁺₂X₁₂ halide perovskites. One novel compound Cs₄MnBi₂Cl₁₂ was synthesised and analysed, with typical d-d transition induced photoluminescence (PL) at 602 nm activated by octahedral coordinated 3d⁵ Mn²⁺.

By manipulating the fraction of B-site vacancies, another novel B-site deficient halide perovskite Cs₁₀MnSb₆Cl₃₀ with unusual one-dimensional 10H (hcccc)₂ structure was synthesised and studied. A similar d-d transition induced PL was observed at 620 nm. However, an enhancement of the quantum yield in comparison with previously reported Cs₄MnSb₂Cl₁₂ suggested that this is due to the different vacancy ordering and reduced dimensionality of octahedral connectivity. Preliminary DFT calculations of the electronic structure of Cs₁₀MnSb₆Cl₃₀ suggested an indirect band gap of 2.69 eV for this compound.

By using azetidinium (Az⁺) as the A-site cation, one novel B-site deficient perovskite polytype with a 6H (hcc)₂ layered structure was observed in Az₃Sb₂Cl₉ and Az₃Sb₂Br₉. The unique vacancy distribution formed an intrinsically polar structure, which was further confirmed by the converse piezoelectric effect. Variable temperature structural analysis and property measurements revealed several phase transitions from the hexagonal prototypical structure to an orthorhombic superstructure between 300 K and 20 K for both compounds. A tuneable band gap and preferential site occupation of chlorine and bromine anions were also observed in the solid-solution Az₃Sb₂Cl₉₋ₓBrₓ.

The other azetidinium based halides were preliminarily investigated. Az₃Sb₂l₉, Az₃Bi₂Br₉ and Az₃Bi₂I₉ showed typical zero-dimensional 6H (hcc)₂ perovskite structure with isolated octahedral dimers; whereas Az₂BiCl₅ and Az₂BiBr₅ were found to adopt non-perovskite cis-connected chain structures.

Date of Award	3 Dec 2025
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Finlay Morrison (Supervisor)