In light of the intense recent interest in the methylammonium lead halides, CHNHPbX (X = Cl, Br, and I) as sensitizers for photovoltaic cells, the dynamics of the methylammonium (MA) cation in these perovskite salts has been reinvestigated as a function of temperature via H,N, and Pb NMR spectroscopy. In the cubic phase of all three salts, the MA cation undergoes pseudoisotropic tumbling (picosecond time scale). For example, the correlation time, τ, for the C-N axis of the iodide salt is 0.85 ± 0.30 ps at 330 K. The dynamics of the MA cation are essentially continuous across the cubic ↔ tetragonal phase transition; however, H andN NMR line shapes indicate that subtle ordering of the MA cation occurs in the tetragonal phase. The temperature dependence of the cation ordering is rationalized using a six-site model, with two equivalent sites along the c-axis and four equivalent sites either perpendicular or approximately perpendicular to this axis. As the cubic ↔ tetragonal phase transition temperature is approached, the six sites are nearly equally populated. Below the tetragonal ↔ orthorhombic phase transition, H NMR line shapes indicate that the C-N axis is essentially frozen.

Mixed-halide lead perovskite (MHP) materials are rapidly advancing as next-generation high-efficiency perovskite solar cells due to enhanced stability and bandgap tunability. In this work, we demonstrate the ability to readily and stoichiometrically tune the halide composition in methylammoniumbased MHPs using a mechanochemical synthesis approach. Using this solvent-free protocol we are able to prepare domain-free MHP solid solutions with randomly distributed halide ions about the Pb center. Up to seven distinct [PbCl x Br 6−x ] 4− environments are identified, based on the 207 Pb NMR chemical shifts, which are also sensitive to the changes in the unit cell dimensions resulting from the substitution of Br by Cl, obeying Vegard's law. We demonstrate a straightforward and rapid synthetic approach to forming highly tunable stoichiometric MHP solid solutions while avoiding the traditional solution synthesis method by redirecting the thermodynamically driven compositions. Moreover, we illustrate the importance of complementary characterization methods, obtaining atomic-scale structural information from multinuclear, multifield, and multidimensional solid-state magnetic resonance spectroscopy, as well as from quantum chemical calculations and long-range structural details using powder X-ray diffraction. The solvent-free mechanochemical synthesis approach is also compared to traditional solvent synthesis, revealing identical solid-solution behavior; however, the mechanochemical approach offers superior control over the stoichiometry of the final mixed-halide composition, which is essential for device engineering.

Mixed-halide lead perovskites are becoming of paramount interest in the optoelectronic and photovoltaic research fields, offering band gap tunability, improved efficiency, and enhanced stability compared to their single halide counterparts. Formamidinium-based mixed halide perovskites (FA-MHPs) are critical to obtaining optimum solar cell performance. Here, we report a solvent-free mechanochemical synthesis (MCS) method to prepare FA-MHPs, starting with their parent compounds (FAPbX; X = Cl, Br, I), achieving compositions not previously accessible through the solvent synthesis (SS) technique. By probing local Pb environments in MCS FA-MHPs using solid-state nuclear magnetic resonance spectroscopy, along with powder X-ray diffraction for long-range crystallinity and reflectance measurements to determine the optical band gap, we show that MCS FA-MHPs form atomic-level solid solutions between Cl/Br and Br/I MHPs. Our results pave the way for advanced methods in atomic-level structural understanding while offering a one-pot synthetic approach to prepare MHPs with superior control of stoichiometry.

We report a comprehensive variable-temperature solid-state 17 O NMR study of three 17 O-labeled crystalline sulfonic acids: 2-aminoethane-1-sulfonic acid (taurine, T), 3-aminopropane-1-sulfonic acid (homotaurine, HT), and 4-aminobutane-1-sulfonic acid (ABSA − groups in these compounds undergo a 3-fold rotational jump mechanism but also extracted the corresponding jump rates (10 2 −10 5 s −1 ) and the associated activation energies (E a ) for this process (E a =4 8± 7, 42 ± 3, and 45 ± 1 kJ mol −1 for T, HT, and ABSA, respectively). This is the first time that SO 3 − rotational dynamics have been directly probed by solid-state 17 O NMR. Using the experimental activation energies for SO 3 − rotation, we were able to evaluate quantitatively the total hydrogen bond energy that each SO 3 − group is involved in within the crystal lattice. The activation energies also correlate with calculated rotational energy barriers. This work provides a clear illustration of the utility of solid-state 17 O NMR in quantifying dynamic processes occurring in organic solids. Similar studies applied to selectively 17 O-labeled biomolecules would appear to be very feasible.

A general correlation for the 129 Xe NMR chemical shift-pore size relationship (δ versus D) in porous silica-based materials over the range 0.5-40 nm has been demonstrated: δ ) δs/(1 + D/b), with δs ) 116 ( 3 ppm and b ) 117 ( 8 Å for the 34 materials studied. The correlation may be used in the characterization of silica samples with unknown pore structure. Even within this general correlation, subsets of materials of similar origin display finer correlations that indicate an acute sensitivity to details of the pore surfaces.The NMR spectroscopy of adsorbed 129 Xe has evolved into a sophisticated scientific tool for the study of different aspects of the structure and topology of internal voids in porous substances. 1 Although some disadvantages of the technique have been noted, 2 it remains attractive and popular, as judged by the fact that in 2001 about 50 Xe NMR-related papers were published. The latest improvements include the production of hyperpolarized xenon (HP Xe), 3 which gives a dramatic increase in the sensitivity for a variety of applications. 4 Besides such practical novelties, important theoretical developments directed at a much needed understanding of chemical shielding phenomena of confined 129 Xe are in progress. 5 For example, through empirical correlations were proposed between isotropic Xe chemical shifts and the pore size (δ-D correlation) in zeolites 6a as well as clathrates and solid Xe, 6b there is still no clear quantitative appreciation of the origin of this behavior. Attempts to extend it beyond zeolites have failed, and it has been pointed out that different correlations must exist for small pores, with a diameter less than about twice the diameter of a Xe atom, and large pores, as in the latter case account must be taken of Xe not adsorbed on the pore walls. 7We have shown previously that a simple fast exchange model explains a qualitatively similar, yet distinct, δ-D correlation found for mesoporous amorphous silica gels with a wide range of mean pore diameters from 2 to 40 nm. 8 Assuming that the 129 Xe NMR chemical shift of xenon adsorbed in mesoporous silica is a dynamic average between the gas and adsorbed states, it is straightforward to derive a parabolic dependence of the 129 Xe NMR chemical shift, δ (ppm), on the mean pore diameter, D (Å), where δ s is the chemical shift characteristic of interactions of Xe with the silica surface, and the parameter b depends on the pore geometry (η), the adsorption constant (K), and the temperature (T), as 8The mean pore diameter (D) is usually given through the volume-to-surface ratio as D ) ηV/S, where the geometry factor η is dependent on the model adopted for the pores. It can vary from 2.8 in a model of randomly packed globular particles (D ) 2.8V/S), to 4 for cylindrical pores (D ) 4V/ S) or 6 for unconnected spherical pores (D ) 6V/S).In this work we show that the model is more generally applicable by extending the range of silica-based porous materials, and we illustrate the correlation with previous work 8, 9 and new results 10 on...