Inversion Symmetry Breaking Probed by X-ray Absorption Spectroscopy in H-bonded Organic Ferroelectric Crystal*

Molecular ferroelectrics (FE) based on ordering hydrogen bonds have potentially high electric polarization and ordering temperature compared with the conventional oxide FE materials. In particular, croconic acid (C₅O₅H₂) with FE polarization of 30 μC/cm², plays a prototype of ferroelectric organic molecular solid, whose electric polarization is generated by proton transfer. We carry out X-ray absorption spectroscopy experiment at oxygen K-edge in croconic acid crystal. The experimental spectrum is well reproduced by the electron-hole excitation theory simulations from configuration generated by ab initio molecular dynamics simulation. When inversion symmetry is broken in ferroelectric state, the hydrogen bonding environment on the two bonded molecules become inequivalent. Such a difference is sensitively probed by the bound excitation in the pre-edge, which are strongly localized on the excited molecules. Our analysis shows that a satellite peak in the pre-edge will emerge at higher excitation energy which serves as a clear signature of ferroelectricity in the material.