Influence of hidden halogen mobility on local structure of CsSn(Cl(1−x)Br(x))(3) mixed-halide perovskites by solid-state NMR

Tin halide perovskites are promising candidates for lead-free photovoltaic and optoelectronic materials, but not all of them have been well characterized. It is essential to determine how the bulk photophysical properties are correlated with their structures at both short and long ranges. Although CsSnCl(3) is normally stable in the cubic perovskite structure only above 379 K, it was prepared as a metastable phase at room temperature. The transition from the cubic to the monoclinic phase, which is the stable form at room temperature, was tracked by solid-state (133)Cs NMR spectroscopy and shown to take place through a first-order kinetics process. The complete solid solution CsSn(Cl(1−x)Br(x))(3) (0 ≤ x ≤ 1) was successfully prepared, exhibiting cubic perovskite structures extending between the metastable CsSnCl(3) and stable CsSnBr(3) end-members. The NMR spectra of CsSnBr(3) samples obtained by three routes (high-temperature, mechanochemical, and solvent-assisted reactions) show distinct chemical shift ranges, spin-lattice relaxation parameters and peak widths, indicative of differences in local structure, defects and degree of crystallinity within these samples. Variable-temperature (119)Sn spin-lattice relaxation measurements reveal spontaneous mobility of Br atoms in CsSnBr(3). The degradation of CsSnBr(3), exposed to an ambient atmosphere for nearly a year, was monitored by NMR spectroscopy and powder X-ray diffraction, as well as by optical absorption spectroscopy.