Hybrid lead halide [(CH3)2NH2]PbX3 (X = Cl- and Br-) hexagonal perovskites with multiple functional properties
We have been able to prepare two new lead halides with the formula DMAPbX3 (where DMA is
dimethylammonium and X is Cl- or Br-), which display a 4H-hexagonal perovskite polytype, an unusual
crystal structure in hybrid organic–inorganic perovskites. Both compounds experience a first-order
phase transition, the chloride at B320 K and the bromide at B250 K, which involves two cooperative
processes: an off-center shift of the lead cations and an order–disorder process of the DMA cations.
Additionally, a sharp dielectric transition is associated with this structural transition in both hybrids. Both
compounds are semiconductors with band gap values of 3.5 eV (X: Cl-) and 3.0 eV (X: Br-). Also, the
LT-phase of the Br compound displays a broad red light photoluminescence (PL) emission (620 nm)
and PLE excitation with the maximum at a soft UV wavelength (360 nm). This broadband emission and
large Stokes-shifted PL seem to be related to a self-trapped exciton mechanism. Therefore, the
uncommon 4H-hexagonal perovskite is a promising crystal structure for understanding and designing
new hybrid materials with broad photoluminescent emission and dielectric properties.