Structure–Band Gap Relationships in Hexagonal Polytypes and Low-Dimensional Structures of Hybrid Tin Iodide Perovskites

Abstract

The present study deals with the structural characterization and classification of the novel compounds 1 – 8 into perovskite subclasses and proceeds in extracting the structure–band gap relationships between them. The compounds were obtained from the employment of small, 3–5-atom-wide organic ammonium ions seeking to discover new perovskite-like compounds. The compounds reported here adopt unique or rare structure types akin to the prototype structure perovskite. When trimethylammonium (TMA) was employed, we obtained TMASnI 3 ( 1 ), which is our reference compound for a “perovskitoid” structure of face-sharing octahedra. The compounds EASnI 3 ( 2b ), GASnI 3 ( 3a ), ACASnI 3 ( 4 ), and IMSnI 3 ( 5 ) obtained from the use of ethylammonium (EA), guanidinium (GA), acetamidinium (ACA), and imidazolium (IM) cations, respectively, represent the first entries of the so-called “hexagonal perovskite polytypes” in the hybrid halide perovskite library. The hexagonal perovskites define a new family of hybrid halide perovskites with a crystal structure that emerges from a blend of corner- and face-sharing octahedral connections in various proportions. The small organic cations can also stabilize a second structural type characterized by a crystal lattice with reduced dimensionality. These compounds include the two-dimensional (2D) perovskites GA 2 SnI 4 ( 3b ) and IPA 3 Sn 2 I 7 ( 6b ) and the one-dimensional (1D) perovskite IPA 3 SnI 5 ( 6a ). The known 2D perovskite BA 2 MASn 2 I 7 ( 7 ) and the related all-inorganic 1D perovskite “RbSnF 2 I” ( 8 ) have also been synthesized. All compounds have been identified as medium-to-wide-band-gap semiconductors in the range of E g = 1.90–2.40 eV, with the band gap progressively decreasing with increased corner-sharing functionality and increased torsion angle in the octahedral connectivity.