3NH₃PbI₃ and CD₃NH₃PbI₃: Quasi-Elastic Neutron Scattering Measurements and First-Principles Analysis Including Nuclear Quantum Effects]]>

摘要

The motion of CH_(3)NH_(3)~(+) cations in the low-temperature phase of the promising photovoltaic material methylammonium lead triiodide (CH_(3)NH_(3)PbI_(3)) 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_(3) against an NH_(3) 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 (CD_(3)NH_(3)PbI_(3)), both theory and experiment observe a higher activation energy for the rotation of CD_(3) against NH_(3), which results from the smaller nuclear quantum effects in CD_(3). The rotation of the NH_(3) 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.