The cubic perovskite structure of black formamidinium lead iodide, α-[HC(NH2)2]PbI3, at 298 K

The structure of black formamidinium lead halide, -[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 while the -NH2 groups hydrogen bond (NH…I = 2.75 – 3.00Å) 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.