Electronic structures and elastic properties of a family of metal-free perovskites

Abstract

The electronic structures and elastic properties of three isostructural, metal-free perovskite materials, (C₄N₂H₁₂)(NH₄X₃)·H₂O (PIP-X, X = Br, Cl, I), were examined using density functional theory (DFT) calculations and high-pressure synchrotron X-ray diffraction experiments. The calculated band structures and density of states demonstrate that all the compounds possess large direct bandgaps of 5.34 eV for PIP-Cl, 4.67 eV for PIP-Br, and 4.13 eV for PIP-I. With the bromide and iodide, the valence band maximum and conduction band minimum mainly arise from the 3p- and 3s-states of the halogens, whereas the conduction band minimum of the chloride is dominated by the s-states of the nitrogen from the ammonium. Such an inverse dependence of bandgaps on the halogen radius originates from the increased band dispersions because of reduced halogen electronegativity. In addition, the full elastic constants of these compounds were calculated using DFT which enables the systematic mapping of their Young's moduli, shear moduli and Poisson's ratios. The N–H⋯X bond strength governed by the halogen radius is primarily responsible for the discrete modulus properties in these compounds. Notably, these metal-free perovskites constructed using hydrogen bonds exhibit comparable rigidity with their hybrid organic–inorganic counterparts assembled using coordination bonds. Furthermore, the high-pressure synchrotron powder X-ray diffraction experiments were performed on PIP-Br, which not only validated the DFT results but also revealed its comparable stiffness to methylammonium lead bromine (CH₃NH₃PbBr₃) under hydrostatic stress.