Compositional variation in hybrid organic-inorganic lead halide perovskites: kinetically- versus thermodynamically-controlled synthesis

The formation and study of partial solid solutions in Az_1-xFA_xPbBr₃, using reportedly similar sized cations azetidinium (Az⁺) and formamidinium (FA⁺), was explored via mechanosynthesis and precipitation synthesis. The composition and lattice parameters of samples from both syntheses were analyzed by ¹H NMR and Rietveld refinement of powder X-ray diffraction. A clear mismatch in the composition of the perovskite was found between the precipitated samples and the corresponding solutions. Such a mismatch was not observed for samples obtained via mechanosynthesis. The discrepancy suggests products are kinetically-controlled during precipitation, compared to thermodynamically controlled mechanosynthesis. Furthermore, the cell volume as a function of composition in both hexagonal, 6H (Az-rich) and cubic, 3C (FA-rich) perovskite solid solutions suggests that FA⁺ is actually smaller than Az⁺, contradicting the literature. In the 3C (Az-poor) solid solutions, the extent of Az_1-xFA_xPbBr₃ is unexpectedly smaller than in the corresponding methylammonium (MA+) system, Az_1-xMA_xPbBr₃, which suggests that the extent of solid solution formation in these halide perovskites is predominantly dependent on the average A-cation size while the size mismatch plays a lesser role in comparison to oxides.