Orientation of Organic Cations in Hybrid Inorganic–Organic Perovskite CH₃NH₃PbI₃ from Subatomic Resolution Single Crystal Neutron Diffraction Structural Studies

Abstract

We report the crystal growth of well-faceted single crystals of methylammonium lead iodide, CH 3 NH 3 PbI 3 , and detailed single crystal neutron diffraction structural studies aimed at elucidating the orientation of the methylammonium (CH 3 NH 3 + ) cation in the tetragonal and cubic phases of the hybrid inorganic−organic perovskite. Room temperature experiments reveal a tetragonal structure where the protonated amine substituent (−NH 3 + ) of the cation is disordered in four positions, each preferentially located near the neighboring iodine of the [PbI 6 ] octahedra, while the methyl substituent (−CH 3 ) is disordered in eight positions located near the body position of the unit cell. High temperature experiments show a cubic structure where the cation aligns along the [011] (edge), the [111] (diagonal), and the [100] (face) directions of the unit cell. The resulting site occupancy ratio suggests the CH 3 NH 3 + cation resides primarily along the [011] direction, in agreement with reported DFT calculations. One important feature that was observed for both tetragonal and cubic structures measured at 295 and 350 K, respectively, is the middle point of the C−N bond being located off-center from the high symmetry sites in the crystal structure, induced by the formation of hydrogen bond-like interactions between the −NH 3 + substituent of the organic cation and the iodine atoms of [PbI 6 ] octahedra.

Molecular orientational melting within a lead-halide octahedron framework: The order-disorder transition in CH3NH3PbBr3

“…We also observe a precipitous increase in the number of active molecules at ∼ 150 K indicative of an increase in local molecular disorder as temperature is increased. This is corroborated by diffraction studies [9][10][11] , NMR studies 51 and computational investigations 17,52 .…”

“…3(b), with the fits from multiple models shown. The expected models to be appropriate based upon previous studies are the three, four and eight site jump models 9,32,41,42 . However, from the results obtained in our experiment, we conclude that the Q-range constrained by kinematics of the experiment is not sufficient to justify the choice of one model above another.…”

“…This radius affects the size of the inorganic cages in which the MA cation sits, and thus the exact radius of rotation accessible for the molecule. In the case of Cl, the decreased size of the halide ion has been linked to the presence of two potential orientational states of the cation in the low temperature phase, as opposed to the single orientation found when I or Br is present instead [7][8][9][10][11] . The choice of halide anion also has an impact on the exact optical properties of the system, and a mixture of anions can be used to tune the band gap of the system 2-5 .…”

“…It is tempting to draw parallels between this and liquid crystals, specifically the melting of the smectic phase to the nematic phase, though in reality the two systems are not very similar 26 . In the nematic phase of liquid crystals there is an orientational order, but no positional order 27,28 whereas in the smectic phase both positional and orientational order is obtained [8][9][10][11] . The situation in MAPbBr 3 is somewhat analogous to the molecular ionic crystals calcite and sodium nitrate where molecular positions are fixed by the crystalline lattice, however transitions from orientational order to disorder occurs with increasing arXiv:1711.07002v1 [cond-mat.mtrl-sci] 19 Nov 2017 temperatures 29,30 .…”

Polar rotor scattering as atomic-level origin of low mobility and thermal conductivity of perovskite CH3NH3PbI3

“…However, the atomic-level description of CH 3 NH 3 PbI 3 is complicated by the hybrid nature where both molecular jumping rotations and phonon excitations have to be taken into account. Moreover, these two components also interact via hydrogen bonds between H and I (refs 18, 19, 20). The best approach for this issue is, without a doubt, inelastic neutron scattering (INS).…”