Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α‑[HC(NH₂)₂]PbI₃, at 298 K

The structure of black formamidinium lead halide, α-[HC­(NH₂)₂]­PbI₃, at 298 K has been refined from high resolution neutron powder diffraction data and found to adopt a cubic perovskite unit cell, a = 6.3620(8) Å. The trigonal planar [HC­(NH₂)₂]⁺ cations lie in the central mirror plane of the unit cell with the formamidinium cations disordered over 12 possible sites arranged so that the C–H bond is directed into a cube face, whereas the −NH₂ groups hydrogen bond (NH···I = 2.75–3.00 Å) with the iodide atoms of the [PbI₃]⁻ framework. High atomic displacement parameters for the formamidinium cation are consistent with rapid molecular rotations at room temperature as evidenced in ab initio molecular dynamic simulations.