Eu2P2S6 exists in centrosymmetric (P21/c) and noncentrosymmetric (Pc) polymorphs whose EuS8 and P2S6 building blocks are virtually identical; the sole distinction is a symmetry breaking assembly governed by weak interactions. Quasi-harmonic free-energy calculations indicate the noncentrosymmetric phase is only ∼0.09 eV per cell above the centrosymmetric ground state, revealing marginal metastability, while first-principle atom response theory predicts a moderate second harmonic generation coefficient (deff ≈ 0.9 pm V–1). Strong 4f electron correlation induces pronounced spin asymmetry, and the same Eu-4f/5d states that dominate linear optics overwhelmingly control the nonlinear response, leading to ionic EuS8 nonlinear optical active domains and covalent, inactive P2S6 counterparts. These results open a spin-selective pathway toward rare-earth nonlinear opto-spintronics.
Xiang et al. (Fri,) studied this question.