Orientational order/disorder and network flexibility in deuterated methylammonium lead iodide perovskite by neutron total scattering

The phase transitions and orientational order of the methylammonium molecular ion in a deuterated sample of [CH3NH3]PbI3 have been studied using neutron total scattering and Reverse Monte Carlo methods. From analysis of the C-N bond orientational distribution functions, obtained both by forming histograms and by analysis using orthonormal spherical functions, we showed that the molecular ions were found to have an almost completely uniform distribution of orientations in the high-temperature cubic phase. The C-N bonds then show progressively increasing order on cooling within the intermediate phase, and complete order in the low-temperature phase. Analysis of the network of connected PbI6 octahedra has shown that the rigidity of the PbI6 octahedra is similar to that in the related cubic perovskite CsPbI3, and is intermediate between the oxide perovskite SrTiO3 and the more flexible ScF3. Both the fluctuations of the I-Pb-I right angle and the orientations of the PbI6 show normal variation with temperature across the range of temperatures of all three phases. The Pb-I bond has been found to be highly anharmonic, comparable to the same bond in CsPbI3. We find no evidence for static ferroelectric distortions at any temperature, but dynamic fluctuations in the local dipole moment are found to be larger than in some other materials. Methylammonium lead iodide crystallises in three phases. The high-temperature phase is cubic with a high degree of orientational disorder of the molecular ions. The intermediate phase shows progressive alignment of the molecules, but still with disorder.