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The mechanisms behind the exceptional photovoltaic properties of themetallorganic perovskites are still debated, and include a ferroelectric (FE)state from the ordering of the electric dipoles of the organic molecules. Wepresent the first anelastic (complex Young's modulus) and new dielectricmeasurements on CH$_{3}$NH$_{3}$PbI$_3$, which provide new insight on thereorientation dynamics of the organic molecules, and the reason why they do notform a FE state. The permittivity is fitted within the tetragonal phase with anexpression that includes the coupling between FE and octahedral tilt modes,indicating that the coupling is competitive and prevents FE ordering. The onsetof the orthorhombic phase is accompanied by sharp stiffening, analogous to thedrop of permittivity, due to the hindered molecular dynamics. On furthercooling, an intense anelastic relaxation process without a dielectriccounterpart suggests the reorientation of clusters of molecules with strongantiferroelectric correlations.

the journal of physical chemistry letters

Competition between Polar and Antiferrodistortive Modes and Correlated  Dynamics of the Methylammonium Molecules in MAPbI$_3$ from Anelastic and  Dielectric Measurements