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Image: see text The structure of black formamidinium lead halide, α-HC(NH(2))(2)PbI(3), 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(NH(2))(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 −NH(2) groups hydrogen bond (NH···I = 2.75–3.00 Å) with the iodide atoms of the PbI(3)(−) 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.
Weller et al. (Tue,) studied this question.