Halogen Modulation and Physical Property Variation in Two Novel Mixed‐Cation Organic‐Inorganic Hybrid Perovskites^†

Comprehensive Summary Organic‐inorganic hybrid perovskites (OIHPs) offer precise control over material properties through the substitution of organic cations and halogens. In this study, two OIHPs, [(C 6 H 10 P)( n ‐BA)]Sb 2 X 9 ([C 6 H 10 P] + = [Me 3 PCH 2 CH 2 CH 3 ] + , [ n ‐BA] + = n ‐butylamine cation, X = I ( 1 ), Br ( 2 )), were synthesized by varying halogens within a mixed‐cation system. The halogen substitution induced a shift from compound 1 , which remains stable without phase transitions, to compound 2 , which undergoes a ferroelastic phase transition ( mmmF 2/ m ). Compound 1 crystallizes in the non‐centrosymmetric Pca 2 1 space group and demonstrates a second harmonic generation (SHG) response, while compound 2 displays elasticity in nanoindentation tests. Halogen substitution alters Sb—X bond lengths, inducing structural distortions in the inorganic framework and influencing the spatial configuration of organic cations and inorganic anions. These changes lead to distinct crystallization behaviors, resulting in different physical properties for compounds 1 and 2 . This work contributes to the development of multifunctional materials based on halogen regulation in mixed‐cation OIHPs.