Spin-Resolved Second-Harmonic Generation from Europium 4f/5d States in a Weakly Stabilized Metastable Polymorph of Eu ₂ P ₂ S ₆

Eu 2 P 2 S 6 exists in centrosymmetric ( P 2 1 / c ) and noncentrosymmetric ( Pc ) polymorphs whose [EuS 8 ] and [P 2 S 6 ] 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 ( d eff ≈ 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 [EuS 8 ] nonlinear optical active domains and covalent, inactive [P 2 S 6 ] counterparts. These results open a spin-selective pathway toward rare-earth nonlinear opto-spintronics.