Flexibility index: a general descriptor of polarization ability in crystalline materials†

Abstract

Although optical phenomena in crystalline materials are different, they are all generated from the polarization processes of microscopic structural units under a photoelectric field. Thus, the development of simple and effective models that can evaluate the polarization ability of optical materials is crucial for their structural screening and mechanism analysis. Herein, we applied the “flexibility (F) index”, a model previously used to investigate the structural origin of second-order nonlinear optical responses in unitary and binary diamond-like materials. We demonstrated that this index has a negative correlation with their band gap and a positive correlation with their first- and second-order polarizabilities. Remarkably, the first-order polarizability showed a linear-function correlation with the F-index, while the second-order polarizability exhibited a cube-function correlation with the “projected-flexibility (F_p) index”. These polarizability-order-dependent correlations validated the feasibility of the F-index in characterizing specific optical properties. The results of this study prove the universality of the F-index to describe optical polarization, which serves as a quick and efficient tool to evaluate the optical response in crystalline materials.