Solid-State Syntheses, Crystallographic Spatial Disorders, Thermal Behavior, and Bandgaps of Hybrid Organic–Inorganic Manganese Halides: A ₂ Mn(Cl/Br) ₄ (A = NH ₄ , C(NH ₂ ) ₃ , & C ₃ H ₄ N ₂ )

By systematically optimizing solid-state synthesis via the concerted use of differential scanning calorimetry and in situ variable-temperature X-ray diffraction, we report the discovery of three new hybrid organic–inorganic halides A 2 Mn(Cl/Br) 4 (A = NH 4 +, C 3 H 4 N 2 + ) and demonstrate that (C(NH 2 ) 3 ) 2 MnBr 4, previously reported from solution-based synthesis, can also be attained from solid-state methods. Overall, changing “A” based on its geometry, polarity, and resonance-stabilized charge delocation influences the crystallographic packing and spatial disorder, thermal behavior, and optical bandgap. With a dipole-free tetrahedral cation, as in (NH 4 ) 2 MnBr 4 ( C 2/ c, mC 180) or (NH 4 ) 2 MnCl 4 ( P 2 1 / c, mP 30), the direct optical bandgaps (4.28–4.36 eV) are wider than those with a trigonal planar dipole-free cation, as in (C(NH 2 ) 3 ) 2 MnBr 4 ( P 2 1 / c, mP 300, 4.07 eV), or a planar dipolar cation geometry, as in (C 3 H 4 N 2 ) 2 MnBr 4 ( I 4 1 / a, tI 384, 4.05 eV). Furthermore, “A” affects the spatial arrangement and dimensionality of the inorganic frameworks comprised of spatially disordered MnBr 4 tetrahedra and/or Mn(Cl/Br) 6 octahedra with varying distortions. These resulting trends, coupled with the systematic investigation into the use of solid-state synthesis, and the motivation behind elucidating the nuanced role of the organic cation, altogether expand the phase space of hybrid materials.