Lattice dynamics and vibrational spectra of the orthorhombic, tetragonal and cubic phases of methylammonium lead iodide

TL;DR

This study uses first-principles calculations to analyze the lattice dynamics and vibrational spectra of different phases of methylammonium lead iodide, revealing mode couplings and instabilities relevant to its structural transitions.

Contribution

It provides detailed phonon spectra and vibrational mode analysis for all phases of CH3NH3PbI3, linking theoretical predictions with experimental data.

Findings

Significant coupling between organic cation and inorganic framework modes.

Identification of soft modes indicating structural instabilities.

Agreement between calculated and experimental vibrational spectra.

Abstract

The hybrid halide perovskite CH3NH3PbI3 exhibits a complex structural behaviour, with successive transitions between orthorhombic, tetragonal and cubic polymorphs at ca. 165 K and 327 K. Herein we report first-principles lattice dynamics (phonon spectrum) for each phase of CH3NH3PbI3. The equilibrium structures compare well to solutions of temperature-dependent powder neutron diffraction. By following the normal modes we calculate infrared and Raman intensities of the vibrations, and compare them to the measurement of a single crystal where the Raman laser is controlled to avoid degradation of the sample. Despite a clear separation in energy between low frequency modes associated with the inorganic PbI3 network and high-frequency modes of the organic CH3NH3+ cation, significant coupling between them is found, which emphasises the interplay between molecular orientation and the corner-sharing octahedral networks in the structural transformations. Soft modes are found at the boundary of the Brillouin zone of the cubic phase, consistent with displacive instabilities and anharmonicity involving tilting of the PbI6 octahedra around room temperature.