Structure–property coupling in lead-free halides Rb3Sb2Br9 and Rb3Sb2Br6I3 obtained by mechanochemistry

Abstract

All-inorganic perovskites exhibit outstanding light absorption properties in the visible range, making them suitable for solar energy applications. We focus on the synthesis of Rb₃Sb₂Br₉ and Rb₃Sb₂Br₆I₃ halides using mechanochemical procedures. Synchrotron X-ray diffraction (SXRD) data were used for determining the crystallographic evolution in the temperature range 80–673 K. Two phases have been identified, the so-called ‘low-temperature polymorph’, with monoclinic symmetry (S.G: P2₁/n), and the ‘high-temperature polymorph’ with trigonal symmetry (S.G: Pm1). At room temperature, SXRD data confirm the presence of a pure monoclinic phase for Rb₃Sb₂Br₆I₃ halide, whereas for Rb₃Sb₂Br₉, there is an admixture with the trigonal phase. The transition temperatures were estimated to be T_t ∼ 376 K and ∼390 K, for Rb₃Sb₂Br₉ and Rb₃Sb₂Br₆I₃, respectively, identified from the DSC curves. The monoclinic P2₁/n structure is composed of double layers of irregular [SbX₆] (X = Br, I) octahedra interconnected through common corners. Each octahedron shares three corners with adjacent octahedra, forming a two-dimensional network, with Rb ions located in the interlayer space. A more symmetric lattice is the trigonal Pm1 polymorph, where the Sb atoms are coordinated in more regular [SbX₆] polyhedra, also forming a 2D network. Symmetry-mode analysis reveals that monoclinic distortion is dominated by octahedral rotations, with cation translations and bond-length modulations acting as secondary stabilizing distortions. Infrared spectroscopy reveals a limited number of active optical phonons, with broader and red-shifted features in Rb₃Sb₂Br₆I₃, indicative of enhanced disorder compared to Rb₃Sb₂Br₉. The obtained values of the direct band gaps from UV-Vis-NIR spectroscopy measurements for the monoclinic phases are ∼2.87 eV (for Rb₃Sb₂Br₉) and ∼2.52 eV (for Rb₃Sb₂Br₆I₃). By correlating local distortions, electronic structure, and thermodynamic stability, our work establishes a microscopic view of phase competition in Rb₃Sb₂X₉ halides and provides design principles for environmentally benign, lead-free optoelectronic materials.