1 research outputs found
Activation Energy of Organic Cation Rotation in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> and CD<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>: Quasi-Elastic Neutron Scattering Measurements and First-Principles Analysis Including Nuclear Quantum Effects
The
motion of CH<sub>3</sub>NH<sub>3</sub><sup>+</sup> cations in the low-temperature phase of
the promising photovoltaic material methylammonium lead triiodide
(CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>) 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<sub>3</sub> against
an NH<sub>3</sub> 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<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub>), both theory and experiment
observe a higher activation energy for the rotation of CD<sub>3</sub> against NH<sub>3</sub>, which results from the smaller nuclear quantum
effects in CD<sub>3</sub>. The rotation of the NH<sub>3</sub> 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