Local Structure and Dynamics in Methylammonium, Formamidinium, and Cesium Tin(II) Mixed-Halide Perovskites from 119Sn Solid-State NMR

Organic–inorganic tin(II) halide perovskites have emerged as promising alternatives to lead halide perovskites in optoelectronic applications. While they suffer from considerably poorer performance and stability in comparison to their lead analogues, their performance improvements have so far largely been driven by trial and error efforts due to a critical lack of methods to probe their atomic-level microstructure. Here, we identify the challenges and devise a 119Sn solid-state NMR protocol for the determination of the local structure of mixed-cation and mixed-halide tin(II) halide perovskites as well as their degradation products and related phases. We establish that the longitudinal relaxation of 119Sn can span 6 orders of magnitude in this class of compounds, which makes judicious choice of experimental NMR parameters essential for the reliable detection of various phases. We show that Cl/Br and I/Br mixed-halide perovskites form solid alloys in any ratio, while only limited mixing is possible for I/Cl compositions. We elucidate the degradation pathways of Cs-, MA-, and FA-based tin(II) halides and show that degradation leads to highly disordered, qualitatively similar products, regardless of the A-site cation and halide. We detect the presence of metallic tin among the degradation products, which we suggest could contribute to the previously reported high conductivities in tin(II) halide perovskites. 119Sn NMR chemical shifts are a sensitive probe of the halide coordination environment as well as of the A-site cation composition. Finally, we use variable-temperature multifield relaxation measurements to quantify ion dynamics in MASnBr3 and establish activation energies for motion and show that this motion leads to spontaneous halide homogenization at room temperature whenever two different pure-halide perovskites are put in physical contact.

Local structure and dynamics in methylammonium, formamidinium and cesium tin(II) mixed-halide perovskites from 119 Sn solid-state NMR Figure S1. Echo-detected 119 Sn MAS NMR spectra of MASnBr3 at 4.7 T, 298 K and 12 kHz MAS using a refocusing echo period of (a) 16.7 μs (asynchronous), (b) 83.3 μs (rotor synchronized). The rotorsynchronized echo period does not lead to the appearance of spinning sidebands and its only effect is lower signal intensity due to fast transverse relaxation. Number of scans: (a) 1024, (b) 4096. Recycle delay: 50 ms. 4 Figure S2. 1 H-13 C CP MAS NMR spectra at 11.7 T, 100 K and 12 kHz MAS of (a) MASnI3, (b) MASnI1.  Figure S4. 133 Figure S5. A plot of the relaxation rate R (R=1/T1) of the variable-temperature T1 relaxation data for MASnBr3 at 16.4 T as a function of the square of temperature. The dependence would be linear if relaxation was dominated by spin-phonon Raman scattering. 3 6 Figure S6. The effect of spinning on 119 Sn NMR spectra and T1 relaxation of tin(II) halide perovskites: (a) FASnI3 (red: static, green: 12 kHz MAS), (b) MASnBr3 (red: static, green 1.5 kHz MAS, blue: 12 kHz MAS), (c) FASnBr3 (red: static, green: 1.5 kHz MAS). The use of MAS does not lead to shortening of the T1 which excludes MAS-induced heteronuclear polarization exchange as a possible relaxation mechanism in MASnBr3. The slight shortening of T1 on going from 1.5 to 12 kHz (panel b) is a temperature-induced effect. 6 Figure S7. WSolids1 (WSolids1 ver. 1.21.3, K. Eichele, Universität Tübingen, 2015) line shape simulations in the extreme narrowing regime of (a) MASnBr3, (b) MASnI3, (c,d) CsSnI3. The relevant simulation parameters are given below the spectra. exp -experimental spectrum, sim -simulated spectrum, J - 119 Sn-X J-coupling, d - 119 Sn-X dipole coupling, T1q -estimated T1 relaxation of the quadrupolar nucleus. Dipolar couplings expected based on the crystal structures (Sn-X distance = 2.9-3.1 Å) are as follows: 119 Sn-81 Br: d = -410 Hz, 119 Sn-79 Br: d = -380 Hz, 119 Sn-127 I: d = -304 Hz. The width of the spectrum is largely determined by the magnitude of J while d introduces a small asymmetry and T1q determines the line broadening in the extreme narrowing regime. In the case of CsSnI3, the asymmetry is reproduced well with a d value larger than the coupling expected based on the crystal structure (panel c). Using the coupling strength based on the crystal structure leads to a more symmetric simulated spectrum (panel d).

List of figures and tables
Since the spectra are echo-acquired, we expect that the slight asymmetry may be an artifact caused by anisotropic 119 Sn T2 relaxation. 7 Figure S8. Powder X-ray diffraction patterns of the materials reported in Figure 2 of the main text.
13 Figure S9. Powder X-ray diffraction patterns of the materials reported in Figure 3 of the main text. The asterisks indicate the corresponding oxidized A2SnX6 phase which slowly forms during the XRD measurement which is carried out in air. 14 Figure S10. Powder X-ray diffraction patterns of the materials reported in Figure 4a-c of the main text. The asterisks indicate the corresponding oxidized A2SnX6 phase which slowly forms during the XRD measurement which is carried out in air. 15 Figure S11. Powder X-ray diffraction patterns of the materials reported in Figure 4d-m of the main text.
15 Figure S12. Powder X-ray diffraction patterns of the materials reported in Figure 5 of the main text. 16 Table S1. Chemical shifts and peak widths of the multi-field variable-temperature T1 relaxation data of MASnBr3, measured using a saturation recovery sequence and a Hahn echo in the quasi-static regime: total echo duration 40 μs at 17.6 and 4.7 T, and 66.7 μs at 9.  Table S3. 119 Sn T1 values measured using a saturation-recovery sequence and fitted using a monoexponential function.  Table S5. Summary of the degradation products of tin(II) halide perovskites detected using 119 Sn       The relevant simulation parameters are given below the spectra. exp -experimental spectrum, sim -simulated spectrum, J - 119 Sn-X J-coupling, d - 119 Sn-X dipole coupling, T1q -estimated T1 relaxation of the quadrupolar nucleus. Dipolar couplings expected based on the crystal structures (Sn-X distance = 2.9-3.1 Å) are as follows: 119 Sn-81 Br: d = -410 Hz, 119 Sn-79 Br: d = -380 Hz, 119 Sn-127 I: d = -304 Hz. The width of the spectrum is largely determined by the magnitude of J while d introduces a small asymmetry and T1q determines the line broadening in the extreme narrowing regime. In the case of CsSnI3, the asymmetry is reproduced well with a d value larger than the coupling expected based on the crystal structure (panel c). Using the coupling strength based on the crystal structure leads to a more symmetric simulated spectrum (panel d). Since the spectra are echo-acquired, we expect that the slight asymmetry may be an artifact caused by anisotropic 119 Sn T2 relaxation.

Supplementary Note 1
It is worth noting that the CSA patterns of MASnCl1.5Br1.5 ( fig. 2f) and MASnBr2.55I0.45 ( fig.  3g) yield a single 119 Sn peak while a sizeable CSA is expected in these cases due to asymmetric coordination of the tin(II) site. Taking a typical value of 119 Sn CSA spanning 100 kHz, the dynamic process which leads to its averaging has to occur at a faster rate, i.e. with a correlation time shorter than 1/(100 kHz) = 10 μs. Since the previously determined halide hopping rates (>0.1 GHz) correspond to correlation times <10 ns, halide hopping might be the source of CSA averaging.

Supplementary Note 2
Estimation of the 119 Sn T1 minimum in the dipole-dipole relaxation mechanism for 119 Sn coupled to 79  ; 1 + ( 01 − 23 ) . . + 3 1 + 01 . . + 6 1 + ( 01 + 23 ) . . @ thermally activated process which is being probed is therefore halide hopping. This confirms that relaxation is driven by halide hopping (i.e. scalar, 1 st kind). 2. Scalar relaxation of the second kind would lead to an activation energy of the process which drives 79/81 Br T1Q, which is typically a Raman process related to lattice modes. Since Raman and phonon spectra fall in the far infrared to THz range, the activation energies corresponding to those processes are <0.03 eV, i.e. an order of magnitude less than what is found experimentally.

Details of NMR measurements
Table S1 . Chemical shifts and peak widths of the multi-field variable-temperature T1 relaxation data of MASnBr3, measured using a saturation recovery sequence and a Hahn echo in the quasi-static regime: total echo duration 40 μs at 17.6 and 4.7 T, and 66.7 μs at 9.4 T. The uncertainties of the monoexponential T1 fit are <1 %.  Table S3. 119 Sn T1 values measured using a saturation-recovery sequence and fitted using a monoexponential function. The uncertainties of the fit are <1 %.

XRD data
Powder X-ray diffraction patterns were recorded on an X'Pert MPD PRO (Panalytical) diffractometer equipped with a ceramic tube (Cu anode, λ = 1.54060 Å), a secondary graphite (002) monochromator and an RTMS X'Celerator (Panalytical) in an angle range of 2θ = 5° to 40°, by step scanning with a step of 0.02 degree.
-S13 - Figure S8. Powder X-ray diffraction patterns of the materials reported in Figure 2 of the main text. Figure S9. Powder X-ray diffraction patterns of the materials reported in Figure 3 of the main text. The asterisks indicate the corresponding oxidized A2SnX6 phase which slowly forms during the XRD measurement which is carried out in air. Figure S10. Powder X-ray diffraction patterns of the materials reported in Figure 4a-c of the main text. The asterisks indicate the corresponding oxidized A2SnX6 phase which slowly forms during the XRD measurement which is carried out in air. Figure S11. Powder X-ray diffraction patterns of the materials reported in Figure 4d-m of the main text. Figure S12. Powder X-ray diffraction patterns of the materials reported in Figure 5 of the main text.