Tuning optical properties and local lone-pair off-centering in “hollow” FA _{1− <i>x</i>} { <i>en</i> } _<i>x</i> Pb _{<i>η</i> − <i>y</i>} Sn _<i>y</i> Br ₃ perovskites

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 FA_1-<i>x</i> en _<i>x</i> Pb _{<i>η</i>-<i>y</i>} Sn _<i>y</i> Br₃ 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.