Abstract
A combined experimental and theoretical approach, consisting of lattice phonon Raman spectroscopy and density functional theory (DFT) calculations, is proposed as a tool for lattice dynamics characterization and polymorph phase identification. To illustrate the reliability of the method, the lattice phonon Raman spectra of two polymorphs of the molecule 2,7-dioctyloxy[1]benzothieno[3,2-b]benzothiophene are investigated. We show that DFT calculations of the lattice vibrations based on the known crystal structures, including many-body dispersion van der Waals (MBD-vdW) corrections, predict experimental data within an accuracy of ≪5 cm–1 (≪0.6 meV). Due to the high accuracy of the simulations, they can be used to unambiguously identify different polymorphs and to characterize the nature of the lattice vibrations and their relationship to the structural properties. More generally, this work implies that DFT-MBD-vdW is a promising method to describe also other physical properties that depend on lattice dynamics like charge transport.
Highlights
U nderstanding and controlling polymorphism is important in materials science due to the significant impact a change in the solid state packing can have on physical properties.[1]
We show that density functional theory (DFT) calculations of the lattice vibrations based on the known crystal structures, including many-body dispersion van der Waals (MBD-vdW) corrections, predict experimental data within an accuracy of ≪5 cm−1 (≪0.6 meV)
This work implies that DFTMBD-vdW is a promising method to describe other physical properties that depend on lattice dynamics like charge transport
Summary
Letter based on force field potentials (FFPs) have been shown to be in qualitative agreement with experimental data (to within 20 cm−1).[12]. Letter of-phase translations of neighboring molecules in the direction of the long molecular axis (Figure 3d), while the latter corresponds to wagging of the molecules perpendicular to the plane of visualization, combined with torsions of the alkyl chains Both sets of vibrations occur in both polymorphs, but only one of them is Raman-active in each polymorph. As a consequence, provided that the crystal structures of different polymorphs are known, DFT-MBD-vdW calculations can be used to calculate lattice phonon Raman patterns at a level of accuracy that allows identification of the phases present in samples characterized by Raman scattering measurements. This offers the possibility to unambiguously identify and locate different polymorph domains within mixed-phase systems.
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