Activation Energy of Organic Cation Rotation in CH₃NH₃PbI₃ and CD₃NH₃PbI₃: Quasi-Elastic Neutron Scattering Measurements and First-Principles Analysis Including Nuclear Quantum Effects

The motion of CH₃NH3⁺ cations in the low-temperature phase of the promising photovoltaic material methylammonium lead triiodide (CH₃NH₃PbI₃) is investigated experimentally as well as theoretically, with a particular focus on the activation energy. Inelastic and quasi-elastic neutron scattering measurements reveal an activation energy of ∼48 meV. Through a combination of experiments and first-principles calculations, we attribute this activation energy to the relative rotation of CH₃ against an NH₃ group that stays bound to the inorganic cage. The inclusion of nuclear quantum effects through path integral molecular dynamics gives an activation energy of ∼42 meV, in good agreement with the neutron scattering experiments. For deuterated samples (CD₃NH₃PbI₃), both theory and experiment observe a higher activation energy for the rotation of CD₃ against NH₃, which results from the smaller nuclear quantum effects in CD₃. The rotation of the NH₃ group, which is bound to the inorganic cage via strong hydrogen bonding, is unlikely to occur at low temperatures due to its high energy barrier of ∼120 meV.