Influence of hidden halogen mobility on local structure of CsSn(Cl1−xBrx)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 CsSnCl3 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 133Cs NMR spectroscopy and shown to take place through a first-order kinetics process. The complete solid solution CsSn(Cl1−xBrx)3 (0 ≤ x ≤ 1) was successfully prepared, exhibiting cubic perovskite structures extending between the metastable CsSnCl3 and stable CsSnBr3 end-members. The NMR spectra of CsSnBr3 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 119Sn spin-lattice relaxation measurements reveal spontaneous mobility of Br atoms in CsSnBr3. The degradation of CsSnBr3, 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.


S7
Supplementary note 1 S7 Table S7. Solid-state 133 Cs NMR acquisition parameters and experimental conditions used for the CsSnBr3 areal degradation study.
S8 Table S8. Solid-state 119 Sn NMR acquisition parameters and experimental conditions used for the CsSnBr3 areal degradation study. Figure S1. Room temperature solid-state 133 Cs NMR spectra of CsSn(Cl1−xBrx)3 materials at 11.75 T with a magic-angle spinning frequency of 13 kHz. Figure S2. Room temperature PXRD (a) and 133 Cs NMR spectra (b) of CsSnCl3 materials prepared by the high-temperature sealed-tube method followed by slow-cooling (5 K/min). PXRD patterns were collected within 24 h of synthesis and 133 Cs NMR were acquired at 11.75 T after six days of synthesis. Plot of 133 Cs NMR peak area for the cubic CsSnCl3 phase as a function of time (up to 72 days) (c). Figure S3. FESEM image and the corresponding EDS elemental mapping for Cs, Sn, Cl and Br for the CsSn(Cl1−xBrx)3 materials. S10 Figure S4. Room temperature experimental and fitted PXRD diagram for the CsSn(Cl1−xBrx)3 series. All the diffraction data are fitted to a cubic (Pm-3m) space group symmetry. S11 Figure S5. Tauc plots showing direct bandgaps of the cubic phases of CsSn(Cl1−xBrx)3 materials. S12 Figure S6. Solid-state 119 Sn NMR spectra of CsSnBr3 at 7.05 and 11.75 T under non-spinning sample conditions. S13 Figure S7. Solid-state 119 Sn NMR spectra of c-CsSnCl3 (a), CsSn(Cl0.50Br0.50)3 (b) and CsSnBr3 (c) at 11.75 T acquired with spinning frequencies of 0 and 10 kHz. S13 Figure S8. Solid-state 119 Sn NMR spectra of CsSn(Cl0.10Br0.90)3 at 11.75 T acquired with spinning frequencies between 0 to 13 kHz with the Hahn-echo pulse sequence and with various echo-delays. S14 Figure S9. Solid-state 119 Sn NMR chemical shifts vs the inverse of direct bandgap values for the CsSn(Cl1−xBrx)3 series. S14 Figure S10. Room temperature PXRD patterns for the CsSnBr3 parent material synthesized by the solvent synthesis (SS), high temperature (HT) and mechanochemical synthesis (MCS) methods. S15 Figure S11. UV-Vis absorption spectra (a) and Tauc plots showing direct bandgaps (b-d) for the CsSnBr3 parent material synthesized by the solvent synthesis (SS), high temperature (HT) and mechanochemical synthesis (MCS) methods. S15 Figure S12. Variable temperature (230-418 K) 119 Sn T2* relaxation time as a function of absolute temperature for the CsSnBr3 (SS) material. S16 Figure S13. UV-Vis absorption spectra for CsSnBr3 that was stored under ambient laboratory conditions over 300 days, pristine CsSnBr3 and Cs2SnBr6. S16 Figure S14. Solid-state 119 Sn NMR spectra of for degraded CsSnBr3 parent and SnBr2 starting precursor at 11.75 T acquired with spinning frequencies of 12 kHz with 2,000,000 scans each.

EXPERIMENTAL Materials
Starting materials were purchased from the following commercial sources and were used without further wt.% in H2O).

High-Temperature Synthesis of CsSn(Cl1−xBrx)3 (0 ≤ x ≤ 1): Various members of the solid solution
CsSn(Cl1−xBrx)3 were prepared by reactions at high temperature. CsX and SnX2 (X = Cl, Br) were combined in stoichiometric ratios on a 0.5-g scale, finely ground using an agate mortar and pestle, pressed into pellets, and loaded into fused-silica tubes which were evacuated under a pressure of 10 -3 mbar and sealed. The tubes were heated at 1.5 K/min to either 673 or 723 K (depending on composition), held at that temperature for 15 h, and then cooled to room temperature at 5 K/min. The samples were stored in glass vials and further characterized under ambient conditions. Cubic CsSnCl3 was obtained as a metastable phase at room temperature as follows: a sample of CsSnCl3 prepared as described above was heated to 673 K, kept at this temperature for 15 h, cooled to 573 K at 5 K/min, and then quenched in an ice-water bath.

Mechanochemical Synthesis of CsSnBr3:
A mixture of 1.5 mmol each of CsBr and SnBr2 was ground using an agate mortar and pestle for 10 min. Within an argon-filled glove box, the mixture was transferred to a 50-mL zirconia grinding vessel (containing ca. 50 g of zirconia balls with 3-8 mm diameter), which was sealed with parafilm to minimize exposure to air. The sample was ground in a Changsha Deco DECO-PBM-V-0.4L electric planetary ball mill at a rotation frequency of 550 rpm for 0.5 h. The vessel was opened to scratch its inner wall by using a clean spatula and sealed again under an inert atmosphere (Ar glove box).
This process was repeated four times for a total of 2.5 h of grinding time.

Solvent Synthesis of CsSnBr3:
A mixture of 1 mmol each of CsBr and SnBr2 was placed in a 40-mL glass vial to which 4.5 mL of concentrated HBr and 0.5 mL of H3PO2 were added.

Energy-dispersive X-ray Spectroscopy and Field Emission Scanning Electron Microscopy:
Samples were examined on Zeiss Sigma 300 VP field emission scanning electron microscope equipped with dual silicon drift detectors for energy-dispersive X-ray spectroscopy to determine chemical compositions.

UV-Visible Diffuse Reflectance Spectroscopy:
Diffuse reflectance spectra were collected on a Cary 5000 UV-vis-NIR spectrophotometer between 200 and 800 nm and calibrated with a Spectralon (>99%) reflectance standard. The diffuse reflectance spectra were converted to absorption spectra using the Kubelka-Munk function, a/S = (1-R) 2 /2R, where a is the Kubelka-Munk absorption coefficient, S is the scattering coefficient, and R is the reflectance. Direct bandgaps were extrapolated from the intercepts in Tauc plots of (ahn) 2 vs E (eV).

Supplementary note 1:
The variable temperature 119 Sn spin-lattice relaxation time (T1) is related to the absolute temperature values for CsSnBr3 (SS) as shown in Table S6. log10[T1/s] linearly depends on [1000/T(K)] (i.e., an Arrhenius relationship) within the temperature range of 230 to 418 K. The slope of the Arrhenius fit is related to the activation energy as Ea = (2303⸱R)⸱(slope), where R = 8.314 J/(mol⸱K). A slope value of 1.51 ± 0.06 K was obtained from the least-squares Arrhenius fit; hence Ea = 28.9 ± 1.2 kJ/mol or 0.30 ± 0.01 eV Table S7. Solid-state 133 Cs NMR acquisition parameters and experimental conditions used for the CsSnBr3 areal degradation study (see Figure 9b in the manuscript). The 133 Cs NMR spectra were acquired at 11.75 T using a Bloch pulse sequence.  Table S8. Solid-state 119 Sn NMR acquisition parameters and experimental conditions used for the CsSnBr3 areal degradation study (see Figure 9c in the manuscript). The 119 Sn NMR spectra were acquired at 11.75 T using a Hahn-echo (νrf = 62.5 kHz).