Synthesis, Crystal and Electronic Structures, and Optical Properties of (CHNH)CdX (X = Cl, Br, I).

We report the synthesis, crystal and electronic structures, as well as optical properties of the hybrid organic-inorganic compounds MACdX (MA = CHNH; X = Cl, Br, I). MACdI is a new compound, whereas, for MACdCl and MACdBr, 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 MACdX 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 MACdCl forms a 2D layered perovskite structure and has two structural transitions at 283 and 173 K. In contrast, MACdBr and MACdI adopt 0D KSO-derived crystal structures based on isolated CdX tetrahedra and show no phase transitions down to 20 K. The contrasting crystal structures and chemical compositions in the MACdX family impact their air stabilities, investigated for the first time in this work; MACdCl is air-stable, whereas MACdBr and MACdI partially decompose when left in air. Optical absorption measurements suggest that MACdX have large optical band gaps above 3.9 eV. Room temperature photoluminescence spectra of MACdX 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 MACdX 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 MACdX. On the basis of our combined experimental and computational results, MACdX and related compounds may be of interest for white-light-emitting phosphors and scintillator applications.