Size Isn’t Everything - Compositional Variation in Hybrid Organic-Inorganic Lead Halide Perovskites: Kinetically- versus Thermodynamically-controlled Synthesis

The formation and study of a partial solid solution Az_1-xFA_xPbBr₃, using ‘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 analysed by ¹H NMR and Rietveld refinement of the 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 6H (Az-rich) and 3C (FA-rich) 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 Az_1-xMA_xPbBr₃, again in contradiction to the expectation based on the reported cation sizes. These results indicate that other factors, as yet unidentified, must also contribute to the solid solution formation of organic-inorganic hybrid perovskites, not simply the relative sizes of the A-site cations.