title: Temperature Evolution of Methylammonium Trihalide Vibrations at the Atomic Scale.
abstract: The temperature evolution of vibrations of CH3NH3PbI3 (MAPI) is studied by combining first principles and classical molecular dynamics and compared to available experimental data. The work has a fundamental character showing that it is possible to reproduce the key features of the vibrational spectrum by the simple physical quantities included in the classical model, namely the ionic-dispersive hybrid interactions and the mass difference between organic and inorganic components. The dynamics reveals a sizable temperature evolution of the MAPI spectrum along with the orthorhombic-to-tetragonal-to-cubic transformation and a strong dependence on molecular confinement and order. The thermally induced weakening of the H-I interactions and the anharmonic mixing of modes give two vibrational peaks at 200-250 cm(-1) that are not present at zero temperature and are expected to have detectable infrared activity. The infrared inactive vibrational peak at ∼140 cm(-1) due to molecular spinning disappears abruptly at the orthorhombic-to-tetragonal transition and forms a broad molecular band red-shifting progressively with temperature. This trend is correlated to the reduced confinement of the rotating cations due to thermal expansion of the lattice.
authors.affiliation: Istituto Officina dei Materiali, CNR-IOM SLACS Cagliari, 09042 Monserrato, Cagliari, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Mattoni
authors.affiliation: Istituto Officina dei Materiali, CNR-IOM SLACS Cagliari, 09042 Monserrato, Cagliari, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Filippetti
authors.affiliation: Istituto Officina dei Materiali, CNR-IOM SLACS Cagliari, 09042 Monserrato, Cagliari, Italy.
authors.forename: M I
authors.initials: MI
authors.lastname: Saba
authors.affiliation: Istituto Officina dei Materiali, CNR-IOM SLACS Cagliari, 09042 Monserrato, Cagliari, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Caddeo
authors.affiliation: Istituto Officina dei Materiali, CNR-IOM SLACS Cagliari, 09042 Monserrato, Cagliari, Italy.
authors.forename: P
authors.initials: P
authors.lastname: Delugas