Crystal and Electronic Structures of New Two Dimensional 3-NH₃-PyPbX₄ Haloplumbate Materials

In this study, we explored both the crystal and electronic structures of new synthesized materials 3-NH₃-PyPbX₄ (X = Br, I). Both compounds are isostructural, and they crystallize in the monoclinic space group P2₁/c, with four formula units in the unit cell. According to the analysis of their electronic structures, both compounds are direct semiconductors with direct transitions between valence and conduction bands occurring at the k-points A, Z, and at about half of the distance between the k-points D/D1 and D1/E. An inspection of DOS reveals that, in both perovskites, the highest energy VBs are comprised mainly of electronic states of halogen anions, while the lowest states in the conduction band originate from lead orbitals. In addition, there are two flat bands composed of electronic states of carbon and nitrogen originating from the organic subsystems and presumably corresponding to the π* orbitals of 3-NH₃-C₅H₆N cations. Both materials demonstrate a broad luminescence emission. Two mechanisms of the radiative relaxation based on either self-trapped excitons or on charge transfers between inorganic and organic subsystems are discussed.