Progressive Polytypism and Bandgap Tuning in Azetidinium Lead Halide Perovskites

Mixed halide azetidinium lead perovskites AzPbBr3-xXx (X = Cl or I) were obtained by mechanosynthesis. With varying composition from Cl- to Br- I-, the chloride and bromide analogues both form in hexagonal 6H polytype while iodide adopts 9R polytype. An intermediate 4H is observed for mixed Br/I compositions. Overall, structure progresses perovskite with composition. Rietveld refinement of powder X-ray diffraction patterns revealed a linear variation unit cell volume as function average radius anion, which not only within solid solution each (according Vegard's law) but also extends uniformly across all three polytypes. This correlated progressive (linear) tuning bandgap 3.43 2.00 eV. Regardless halide, family polytypes are highly stable, no discernible change properties over more than 6 months under ambient conditions.