# Tuning optical properties and local lone-pair off-centering in “hollow” FA1−x{en}xPbη−ySnyBr3 perovskites

## TL;DR

This paper explores hollow perovskites to show how their optical properties and structure can be tuned by adjusting composition and local distortions.

## Contribution

The study demonstrates control over optical properties and local lone-pair off-centering in hollow perovskites through compositional tuning.

## Key findings

- Optical gaps in FA1−xenxPbη−ySnyBr3 perovskites range from 1.9 eV to 2.6 eV.
- Local off-centering distortions correlate with Sn concentration and are regulated by ethylenediammonium dications.

## Abstract

Hollow metal halide perovskites, while a relatively recent discovery, have already been shown to push the boundaries of tuneability within an inherently limited compositional space. Here, we further expand our knowledge of these complex materials demonstrating the ability to control optical properties and local structure, notably lone-pair induced off-centering. We focus on the mixed metal FA1−xenxPbη−ySnyBr3 hollow perovskites. These compounds have wide ranging optical gaps from 1.9 eV (deep red) to 2.6 eV (light yellow), combining the anomalous bandgap red-shifting of Pb/Sn mixing with the blue-shifting effects of the organic substitution. Average and local structural studies employing single crystal X-ray diffraction and total X-ray scattering pair distribution function analyses respectively suggest strong incoherent off-centering distortions that are locally correlated with Sn concentration. The inclusion of ethylenediammonium dications appears to regulate metal off-centering, opening new opportunities of research into this phenomenon.

The synthesis, structural, and optoelectronic characterization of the family of FA1−xenxPbη−ySnyBr3 hollow metal halide perovskites highlighting the interplay between the local structure dynamics and optical properties.