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-x)FA(x)PbBr(3), using reportedly similar-sized cations azetidinium (Az(+)) and formamidinium (FA(+)), were explored via mechanosynthesis and precipitation synthesis. The compositions and lattice parameters of samples from both syntheses were analyzed by H-1 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 that 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-x)FA(x)PbBr(3) is unexpectedly smaller than that in the corresponding methylammonium (MA(+)) system, Az(1-x)FA(x)PbBr(3), 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.