Polytypism, which is ubiquitous in oxide perovskites, also shows up in halide perovskites. Polytype structures have been experimentally observed using a variety of techniques and their influence on carrier performance has also been characterised (details can be found in the bottom reference papers). There are fascinating electronic and ionic phenomena to be expected in these materials.

Typical halide polytypes are illustrated here:

This repository contains a set of tools and crystal structures for the description of perovskite polytypes in general, but halide perovskites in particular. We provide a combined method of Ising-type model Hamiltonian and genetic algorithm, which can be used to identify low energy configurations.

It is structured as follows:

A python code to generate arbitrary perovskite stacking sequences to access complex polytypes and stacking faults.
Tips for generating the disp for any polytypes: compare the sequence with a perfect 3C structure, if the positions of both AX₃ and B remain the same, then the index is (0, 0); for movement of AX3 and B, moving towards left (index + 1), moving towards right (index -1).

Scripts for electrostatic analysis.

Python codes to search for high or low energy stacking sequences. See the IsingHamilotian_GA.ipynb file.

3D structure models that have been generated for various perovskites. They may be in VASP format (POSCAR) or Crystallographic Information File (CIF). Both can be opened using VESTA.

The main language is Python 3 and has been tested using Python 3.6+. Basic requirements are Numpy and Scipy. pymatgen is also required for some of the tools.

• Ramsdell notation for polytypes

• The Many Faces of Mixed Ion Perovskites: Unraveling and Understanding the Crystallization Process

• Hexagonal stacking faults act as hole-blocking layers in lead halide perovskites (ACS Energy Letters, 2020)

• Evolutionary exploration of polytypism in lead halide perovskites (Chemical Science, 2021)

• Inhomogeneous defect distribution in mixed-polytype metal halide perovskites (ACS Energy Letters, 2023)

• From cubic to hexagonal: electronic trends across metal halide perovskite polytypes (Journal of Physical Chemistry C, 2023)