Cubic or Not Cubic? Combined Experimental and Computational Investigation of the Short-Range Order of Tin Halide Perovskites

Abstract

Tin-based metal halide perovskites with a composition of ASnX₃ (where A= MA or FA and X = I or Br) have been investigated by means of X-ray total scattering techniques coupled to pair distribution function (PDF) analysis. These studies revealed that that none of the four perovskites has a cubic symmetry at the local scale and that a degree of increasing distortion is always present, in particular when the cation size is increased, i.e., from MA to FA, and the hardness of the anion is increased, i.e., from Br^- to I^-. Electronic structure calculations provided good agreement with experimental band gaps for the four perovskites when local dynamical distortions were included in the calculations. The averaged structure obtained from molecular dynamics simulations was consistent with experimental local structures determined via X-ray PDF, thus highlighting the robustness of computational modeling and strengthening the correlation between experimental and computational results.

Figure 1

Overlay of the raw X-ray PDF data collected from 300 to 360 K from ASnX₃ where A = MA or FA and X = I or Br: (A) MASnI₃, (B) FASnI₃, (C) MASnBr₃, and (D) FASnBr₃.

Figure 2

Fits of the X-ray PDF data at 300 K from 2.0 to 5.0 Å against the various space groups for (A) MASnI₃ and (B) FASnI₃: Pm3̅m, R3m, and I4cm. Gray dotted line, observed; red line, calculated; blue line, difference (shifted by −2.5 for ease of visualization).

Figure 3

Fit of the X-ray PDF data at 300 K from 2.0 to 5.0 Å for (A) MASnBr₃ and (B) FASnBr₃ against the orthorhombic models. Gray dotted line, observed; red line, calculated; blue line, difference (shifted by −2.5 for ease of visualization).

Figure 4

Structural sketches of the models selected to describe the local structure of ASnX₃, where A = MA or FA and X = I or Br: MASnI₃, MASnI₃, MASnBr₃, and FASnBr₃.

Figure 5

Raw X-ray PDF data collected at 80 K from ASnX₃, where A = MA or FA and X = I or Br.

Figure 6

(A) Fit of the X-ray PDF data at 80 K from 2.0 to 5.5 Å against space group Pnma for FASnI₃. (B) Fit of the X-ray PDF data at 80 K from 2.0 to 5.0 Å for MASnBr₃ against the orthorhombic model. Gray dotted line, observed; red line, calculated; blue line, difference (shifted by −2.5 for ease of visualization).

Figure 7

Comparison between X-ray PDF data collected at RT and MD simulations for ASnX₃, where A = MA or FA and X = I or Br). Gray dotted line, observed; red line, calculated; blue line, difference (shifted by −2.5 for ease of visualization).