| Variable-temperature MAS NMR | Linewidth and relaxation analyses yield consistent hopping barriers and an Arrhenius-extrapolated room-temperature bulk halide diffusivity of order 10⁻¹¹ cm² s⁻¹. Because the extrapolation spans structural phase transitions, the value is treated as an order-of-magnitude estimate rather than a direct room-temperature measurement |
|---|---|
| Variable-temperature MAS NMR composition series | Variable-temperature MAS NMR was used for CsPbBr₃, CsPb(Br₀.₅I₀.₅)₃, (Cs₀.₇₅Rb₀.₂₅)PbBr₃, and (Cs₀.₇₅Rb₀.₂₅)Pb(Br₀.₅I₀.₅)₃ |
| MAS NMR acquisition | Variable-temperature ¹³³Cs and ²⁰⁷Pb MAS NMR spectra were acquired at 20.0 T and 4 kHz MAS |
| ¹³³Cs MAS NMR phase-transition signatures | For CsPbBr₃, the temperature dependence of the isotropic ¹³³Cs shift and linewidth reveals two known phase transitions and supports the existence of an additional low-temperature transition previously proposed from diffraction. With compositional mixing, static disorder substantially broadens the room-temperature ¹³³Cs resonances, masking some spectral discontinuities that would otherwise mark phase transitions directly; the broadening reflects a distribution of local environments created by A-site or X-site substitution |
| ¹³³Cs spin-lattice relaxation | In all four compositions, ¹³³Cs spin-lattice relaxation time T₁ reveals distinct minima associated with structural phase transitions, although these are not observed for every phase transition. For the three materials in which the high-temperature transition can already be assigned from one-dimensional spectra, the transition temperatures coincide with T₁ minima. For (Cs₀.₇₅Rb₀.₂₅)Pb(Br₀.₅I₀.₅)₃, where static disorder makes straightforward spectral assignment difficult, T₁ clearly captures the high-temperature transition |
| ²⁰⁷Pb MAS NMR of CsPbBr₃ | Variable-temperature ²⁰⁷Pb MAS NMR provides an early indication of rapidly changing dynamics: scalar (J) coupling between lead and bromine disappears at elevated temperatures, indicating loss of a static Pb–Br bonding environment on the NMR timescale |
| ¹³³Cs motional narrowing and hopping barriers | For CsPb(Br₀.₅I₀.₅)₃, the ¹³³Cs signal is broad at room temperature and narrows very substantially on heating, yielding an activation energy for halide hopping of about 0.34 eV by linewidth analysis. For (Cs₀.₇₅Rb₀.₂₅)PbBr₃, A-site disorder broadens the low-temperature signal, but narrowing remains incomplete even at the highest temperatures measured, indicating that A-site diffusion is still comparatively slow. For (Cs₀.₇₅Rb₀.₂₅)Pb(Br₀.₅I₀.₅)₃, the much broader room-temperature ¹³³Cs line undergoes nearly complete narrowing at high temperature in two regimes corresponding to two independently resolved motional averaging processes. The lower-temperature regime yields the same halide activation energy found for CsPb(Br₀.₅I₀.₅)₃, suggesting that halide diffusion is largely insensitive to A-site composition; the higher-temperature regime yields an A-site hopping barrier of around 0.8 eV, attributed to motion in the A-site sublattice. These observations collectively show that in this system halide mixing accelerates both halide and A-site diffusion |
| High-temperature ¹³³Cs T₁ minimum | In the two mixed-halide compositions, but not in the bromide-only materials, ¹³³Cs T₁ exhibits a second high-temperature minimum that does not correspond to a phase transition. For ¹³³Cs at 20 T, this minimum corresponds to motion on the nanosecond timescale and is most consistently attributed to halide diffusion. Its appearance at nearly the same temperature in CsPb(Br₀.₅I₀.₅)₃ and (Cs₀.₇₅Rb₀.₂₅)Pb(Br₀.₅I₀.₅)₃ reinforces the linewidth-analysis conclusion that halide diffusion depends primarily on X-site rather than A-site composition |
| ¹³³Cs spin-spin relaxation | In (Cs₀.₇₅Rb₀.₂₅)Pb(Br₀.₅I₀.₅)₃, T₂′ exhibits a lower- and a higher-temperature minimum, attributed respectively to halide hopping and A-site cation hopping. The higher-temperature minimum yields an activation energy of 0.77 eV, close to the 0.84 eV obtained from linewidth analysis. In CsPb(Br₀.₅I₀.₅)₃, only the lower-temperature minimum is observed within the accessible temperature window |
Benjamin M. Gallant, Priyanka Deswal, Shrestha Banerjee, Erik Fransson, Jem Hammerton, Michael Walters, Joshua Deakin, William Burston, Paul Erhart, Julia Wiktor, and Dominik J. Kubicki. “Compositional disorder controls bulk ion diffusion in mixed-ion halide perovskites.” ChemRxiv (). https://doi.org/10.26434/chemrxiv.15008055/v1
No separate Supporting Information file was available on the ChemRxiv article page when acquired