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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.