Synthesis, Crystal and Electronic Structures, and Optical Properties of (CH₃NH₃)₂CdX₄ (X = Cl, Br, I)

We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MA₂CdX₄ (MA = CH₃NH₃; X = Cl, Br, I). MA₂CdI₄ is a new compound, whereas, for MA₂CdCl₄ and MA₂CdBr₄, structural investigations have already been conducted but electronic structures and optical properties are reported here for the first time. Single crystals were grown through slow evaporation of MA₂CdX₄ solutions with optimized conditions yielding mm-sized colorless (X = Cl, Br) and pale yellow (X = I) crystals. Single crystal and variable temperature powder X-ray diffraction measurements suggest that MA₂CdCl₄ forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MA₂CdBr₄ and MA₂CdI₄ adopt 0D K₂SO₄-derived crystal structures based on isolated CdX₄ tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MA₂CdX₄ family impact their air stabilities, investigated for the first time in this work; MA₂CdCl₄ is air-stable, whereas MA₂CdBr₄ and MA₂CdI₄ partially decompose when left in air. Optical absorption measurements suggest that MA₂CdX₄ have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MA₂CdX₄ yield broad peaks in the 375-955 nm range with full width at half-maximum values up to 208 nm. These PL peaks are tentatively assigned to self-trapped excitons in MA₂CdX₄ following the crystal and electronic structure considerations. The bands around the Fermi level have small dispersions, which is indicative of high charge localization with significant exciton binding energies in MA₂CdX₄. On the basis of our combined experimental and computational results, MA₂CdX₄ and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.