Strong Piezoelectric Response in an Oxalate-Coordinated Aluminum Crystal NaBaAl(C ₂ O ₄ ) ₃ ·5H ₂ O

A novel oxalate-based piezoelectric crystal, NaBaAl(C₂O₄)₃·5H₂O, was successfully synthesized and structurally characterized. The compound crystallizes in the polar space group <i>Pca</i>2₁, forming a robust three-dimensional framework constructed from [Al(C₂O₄)₃]^3- units stabilized by Na⁺, Ba²⁺, and coordinated water molecules. Optical diffuse reflectance spectroscopy combined with Tauc's analysis indicates a direct bandgap of 4.53 eV, while polarized light microscopy measurements yield an experimental moderate birefringence of 0.044, confirming significant optical anisotropy. More notably, piezoelectric measurements reveal a relatively large coefficient of 67.9 pC/N at room temperature, representing an order-of-magnitude enhancement compared with previously reported Al-based piezoelectric crystals. Furthermore, variable-temperature measurements demonstrate the evolution of the piezoelectric response with heating. In addition, PFM measurements confirm polarization switching under applied bias, supporting the piezoelectric assignment. Collectively, these results expand the family of Al-based piezoelectrics and highlight the potential of ligand-engineered frameworks as a viable strategy for developing high-performance piezoelectric materials.