B-site substitution effects on the mechanical properties of halide perovskites [C4H12N2][BCl3]·H2O (B = NH4+; K+)

Abstract

The mechanical properties of halide perovskites have been attracting ever-increasing interest for their significant importance in future industrial applications. However, studies focused on the effect of B-site substitution of molecular perovskites on their mechanical properties are rare, which makes it favorable to shed light on their fundamental structure–mechanical property relationships. Here, using isostructural halide perovskites, [C₄H₁₂N₂][BCl₃]·H₂O (B = NH₄⁺; K⁺), constructed by ionic bonds and hydrogen bonds, respectively, as the model systems, we investigate their mechanical properties through high-pressure synchrotron X-ray diffraction experiments and density functional theory calculations. Owing to the similar sizes of NH₄⁺ and K⁺, the two compounds possess almost identical cell parameters and frameworks. Upon compression, the two perovskites exhibit analogous behavior except for slight differences in the shrinkage ratio of principal axes and the onset pressure of amorphization. The fitted bulk moduli of [C₄H₁₂N₂][KCl₃]·H₂O and [C₄H₁₂N₂][NH₄Cl₃]·H₂O are 43.89 and 27.28 GPa, respectively. These results demonstrate that the simple replacement of K⁺ by NH₄⁺ can significantly reduce the structural rigidity of the corresponding compounds, which is ascribed to the weaker strength of NH₄⋯Cl hydrogen bonds than that of K–Cl bonds.