Piezoelectricity from Dopant-Induced Structural Distortions in Molecular Crystals Revealed by Raman Spectroscopy

Area of Science:

• Solid-state chemistry
• Materials science
• Crystal engineering

Background:

• Crystal engineering enables functional molecular solids via controlled noncovalent interactions.
• Centrosymmetric crystals can exhibit piezoelectricity through stereoselective doping with chiral additives.
• The microscopic origin of polarization in doped crystals (dipole mismatch vs. lattice distortion) is often unclear.

Purpose of the Study:

• To disentangle the contributions of dipole mismatch and lattice distortion to piezoelectricity in doped molecular crystals.
• To establish a direct structure-function relationship in these materials.
• To provide design principles for engineering electromechanical responses.

Main Methods:

• Incorporation of four chiral N-acetyl-L-amino acids into N-acetyl-DL-valine crystals.
• Piezoelectric measurements.
• Density functional theory (DFT) calculations.
• Low-frequency Raman spectroscopy to detect symmetry-breaking phenomena.

Main Results:

• Macroscopic piezoelectricity directly correlates with the magnitude and orientation of local lattice distortions.
• Dipole mismatch along the polar axis plays a secondary role in the studied systems.
• Low-frequency Raman spectroscopy revealed concentration-dependent symmetry-breaking not seen in X-ray diffraction.

Conclusions:

• Local lattice distortions are the primary drivers of piezoelectricity in these doped molecular crystals.
• A direct structure-function relation is established for piezoelectric response.
• Biased local distortions offer a predictive design strategy for electromechanical materials.