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

The motion of CHNH cations in the low-temperature phase of the promising photovoltaic material methylammonium lead triiodide (CHNHPbI) 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 CH against an NH group that stays bound to the inorganic cage.