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Mechanochemical Synthesis of Methylammonium Lead Mixed–Halide Perovskites: Unraveling the Solid-Solution Behavior Using Solid-State NMR
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 methylammonium-based 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<sub><i>x</i></sub>Br<sub>6–x</sub>]<sup>4–</sup> environments are identified,
based on the <sup>207</sup>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