Revealing the Local Sn and Pb Arrangements in CsSn_xPb_1–xBr₃ Perovskites with Solid-State NMR Spectroscopy

Mixed Sn–Pb halide perovskites are more stable at ambient conditions and can be tuned to give narrower band gaps than the all-Pb-containing counterparts (APbX3) used as photovoltaic materials. In the series CsSnxPb1–xBr3, the crystal structure evolves from orthorhombic (space group Pnma for x = 0–0.8) to cubic (space group Pm3¯m for x = 1), and the band gap decreases for Sn-richer compositions. It previously was unclear how the physical properties are related to structural changes entailed by the Sn–Pb mixing because the short- versus long-range arrangements have not been well characterized. Solid-state NMR spectroscopy of several NMR-active nuclei (119Sn, 133Cs, and 207Pb) supports the occurrence of complete disorder, with PbBr6 and SnBr6 octahedra distributed in random arrangements throughout the entire structure with no evidence for phase segregation. Compounds prepared by solvent-assisted vs solvent-free routes are compared and show differences in their degree of crystallinity and optical absorption properties.