Mixed Triple-Cation Tin and Lead Bromide Perovskites: Bridging Atomic Structure, Lattice Dynamics, and Stability

Abstract

Tin-based halide perovskites are promising lead-free alternatives for optoelectronic applications, yet their practical implementation is limited by the facile oxidation of Sn(II) to Sn(IV), which compromises environmental stability. Inspired by the success of triple-cation engineering in Pb-based perovskites, we explored the mixed A-site series FA_0.80MA_0.20–xCs_xSnBr₃ (x = 0–0.20) and the corresponding Pb analogues, synthesized via a solvent-free mechanochemical route. Powder X-ray diffraction confirmed the formation of phase-pure perovskite compounds, while diffuse reflectance spectroscopy showed only minor changes in the optical band gap (E_g) upon A-site mixing. Multinuclear solid-state nuclear magnetic resonance (NMR) spectroscopy, including high-resolution two-dimensional double-quantum single-quantum ¹H NMR spectra, demonstrated homogeneous mixing of formamidinium (FA), methylammonium (MA), and cesium cations without phase segregation or clustering. Quantification of ¹H and ¹³C NMR spectra confirmed the intended cationic compositions, whereas ¹³³Cs, ¹¹⁹Sn, and ²⁰⁷Pb NMR spectra revealed the influence of cation mixing on the local chemical environments. Comparative ⁷⁹Br nuclear quadrupolar resonance spectroscopy spectra on the Pb analogues further supported a random cation distribution. Ambient stability studies showed that Cs₂SnBr₆ is the primary degradation product of the Sn series and that moderate Cs incorporation partially improves structural stability, while higher loadings promote degradation. Finally, time-domain terahertz spectroscopy revealed distinct lattice-dynamical responses in the Sn and Pb series, indicating stronger lattice polarization in the Pb compounds and lattice stiffening accompanied by partial stabilization in the Sn compounds upon Cs incorporation. These results establish a direct correlation among homogeneous A-site mixing, local structure, lattice dynamics, and stability, providing mechanistic insight into the design of stable lead-free tin halide perovskites.