A Critical Assessment on Calculating Vibrational Spectra in Nanostructured Materials.

Area of Science:

• Materials Science
• Computational Chemistry
• Spectroscopy

Background:

• Vibrational spectroscopy is crucial for characterizing functional nanostructured materials like zeolites, MOFs, and MHPs.
• Experimental spectra are complex, often requiring theoretical calculations for full elucidation.
• Existing theoretical approaches include static (harmonic oscillator approximation) and dynamic (molecular dynamics) methods.

Purpose of the Study:

• To provide a comprehensive comparison of static and dynamic theoretical methods for predicting vibrational spectra of nanostructured materials.
• To establish theoretical guidelines for selecting the appropriate methodology based on material properties and conditions.
• To address the lack of comparative studies and guide researchers in lattice dynamics studies.

Main Methods:

• Utilized static and dynamic approaches to calculate vibrational spectra for four distinct nanostructured materials.
• Investigated case studies including flexible MOFs, defective MOFs (UiO-66), metal-halide perovskites (CsPbBr3), and zeolites (H-SSZ-13).
• Compared theoretical results with experimental spectra under varying conditions (temperature, presence of defects/guests).

Main Results:

• At low temperatures and in defect/guest-free states, both static and dynamic methods provide similar, qualitatively correct spectra.
• The static (harmonic) approach fails at higher temperatures for materials like CsPbBr3 due to anharmonic phonon modes.
• Defects and guest species significantly impact spectra, necessitating dynamic sampling for accurate prediction, as the harmonic model proves insufficient.

Conclusions:

• For crystalline framework materials at low temperatures, static methods suffice for lattice dynamics insights.
• Dynamic sampling is required for accurate phonon spectrum prediction at higher temperatures or when anharmonicity, defects, or guest species are present.
• The proposed guidelines aid researchers in selecting optimal theoretical methodologies for vibrational spectra prediction in functional solid-state materials.