Structural\nTransition in (C₂H₅NH₃)₃Bi_2–<i>x</i>Sb_<i>x</i>I₉:[(Bi/Sb)₂I₉]^3– Dimers to [(Bi/Sb)₃I₁₂]^3– Trimers to (∞^<b>1</b>)[(Bi/Sb)₂I₉^3–] 1D Infinite Chains

Abstract

Antimony/bismuth-based lead-free hybrid halide defect\n2D perovskites\nhave been generating enormous research interest due to their inherent\nexcellent optical properties. Exploration of new phases and understanding\nof their structural and optoelectronic properties are of paramount\nimportance in the process of developing materials for practical solar\ncell applications. In this article, we have reported a structural\ntransition from the 0D hexagonal phase containing isolated [M₂I₉]^3– (M = Bi/Sb) units to the\n1D orthorhombic phase via a new monoclinic phase with novel isolated\ntrimeric [M₃I₁₂]^3– units in\n(C₂H₅NH₃)₃Bi_{2–2<i>x</i>}Sb_2<i>x</i>I₉. The\nhexagonal phase is stable up to 2<i>x</i> = 0.6 in (C₂H₅NH₃)₃Bi_{2–2<i>x</i>}Sb_2<i>x</i>I₉. With\ngradual substitution of Sb, the cation–cation repulsion increases,\nwhich destabilizes the [M₂I₉]^3– unit, and hence, the hexagonal phase becomes unstable. At intermediate\ncomposition, 2<i>x</i> = 0.8–1.6, a new monoclinic\nphase (S.G.: <i>C</i>2/<i>m</i>) with the composition\n(C₂H₅NH₃)₂Bi_{2–2<i>x</i>}Sb_2<i>x</i>I₈ is\nformed, containing isolated [M₃I₁₂]^3– units. The symmetry reduction resulted in larger distortion, which\nrelaxes the strain and stabilizes the trimeric unit in the intermediate\ncompositions. Finally, at higher Sb compositions (2<i>x</i> = 1.9–2.0), the compounds crystallize in the orthorhombic\n1D phase. In all three phases of (C₂H₅NH₃)₃Bi_{2–2<i>x</i>}Sb_2<i>x</i>I₉, the cationic ethylammonium\nunits are completely disordered over the whole unit cell. Raman study\nclearly shows the phase transition in (C₂H₅NH₃)₃Bi_{2–2<i>x</i>}Sb_2<i>x</i>I₉ and also the structural distortion\nin (C₂H₅NH₃)₂Bi_{2–2<i>x</i>}Sb_2<i>x</i>I₈. Optical\nproperty study shows that all the compounds are of indirect band gap\ntype. Furthermore, PL study shows better emission properties of the\n1D orthorhombic Sb compounds as compared to the 0D hexagonal and monoclinic\nphases.