Synthesis and dispersion-corrected density functional theory calculations of formamidinium-indium-iodide/chloride single crystals

Single crystals of formamidinium (FA)–indium (In)–iodide (I) and chloride (Cl) were synthesized to explore the functionality in hybrid halides involving cations with a d¹⁰ electronic configuration. The chemical formulae of the iodide and chloride were determined to be FAInI₄ and FA₃InCl₆, respectively, identifying them as trivalent In (4d¹⁰) compounds. Single-crystal X-ray diffraction at room temperature resolved the arrangements of [InI₄]^- tetrahedra and FA ions in FAInI₄, which crystallized in a monoclinic (P2₁/n) cell. In contrast, FA₃InCl₆ exhibited a transformation to a superlattice structure at lower temperatures; its atomic arrangements were almost entirely resolved at 150 K but not clearly at 200 K or higher. Furthermore, dispersion-corrected density functional theory calculations employed to determine the detailed arrangements of FA ions indicated that these crystal structures were stabilized by the formation of hydrogen bonds between FA ions and the inorganic components. The broad-band visible luminescence in FA₃InCl₆ and the complete luminescence quenching observed in FAInI₄, are discussed in terms of interactions between FA ions and the inorganic polyhedra.