Cation and anion co-partial substitution induced centrosymmetric to noncentrosymmetric structural transformation to construct nonlinear-optical rare-earth oxythiogermanates

Abstract

Rational structural design and modification based on known compounds is an effective strategy to explore new nonlinear optical (NLO) materials. Here, three new rare-earth (RE) oxythiogermanates Eu₁₈Ge₉O₅S₃₁ (1), Ca_3.32Eu_14.68Ge₉O₅S₃₁ (2) and Ba₃Eu₁₅Ge₉O₅S₃₁ (3) were obtained by the novel cation and anion co-partial substitution strategy from the parent β-Eu₂GeS₄ (0), which induces centrosymmetric to noncentrosymmetric (NCS) structural transformation and NLO activities. They crystallize with a pseudo-zero dimensional structure in the chiral R3 space group, featuring almost linearly arranged isolated GeS₄ and GeOS₃ tetrahedra. Their optical band gaps were determined to be 2.18, 2.23 and 2.24 eV, respectively. 1 and 2 show balanced NLO properties with moderate second-harmonic generation (SHG) responses (0.5 and 0.6 × AGS) and high laser-induced damage thresholds (LIDTs) (3.3 × AGS). Moreover, theoretical calculations show that their NLO properties are determined by the synergistic effect of GeS₄ and GeOS₃ tetrahedra. This work enriches the chemistry of rarely studied RE oxychalcogenides and provides a fresh route for designing NCS structures targeting diverse applications.