The first Hg-based oxychalcogenide Sr2HgGe2OS6: Achieving balanced IR nonlinear optical properties through synergistic cation and anion substitution

Abstract

Recently, newly emerging Hg-based chalcogenides with fascinating non-centrosymmetric (NCS) structures and superior optical properties have received wide attention as candidates for infrared nonlinear optics (IR-NLO). However, simultaneously optimizing three key performance indicators in these materials, namely the second-harmonic generation coefficient (deff), optical energy gap (Eg), and birefringence (Δn), remains a major challenge due to their often-competing structural requirements. Herein, the first Hg-based oxychalcogenide, Sr2HgGe2OS6, was rationally obtained using a synergistic strategy of cation and anion design based on the well-known melilite-type oxide Sr2ZnGe2O7. By substituting Hg2+ for Zn2+ cations and S2− for O2− anions, Sr2HgGe2OS6 features new 2D Cairo pentagonal tiling layers of [HgGe2OS6]4–, separated by charge-balanced Sr2+ cations. Notably, it exhibits well-balanced IR-NLO properties with a substantial deff (1.2 × AgGaS2@2050 nm), wide Eg (3.25 eV) resulting in a favorable laser-induced damage threshold (15.6 × AgGaS2@1064 nm), broad transparent window (up to 14.3 μm), and sufficient calculated Δn (0.147@2050 nm) for phase-matching ability across a wide range. Moreover, detailed theoretical analyses suggest that the simultaneous presence of excellent linear and NLO properties in Sr2HgGe2OS6 can be attributed to the cooperative effect of heteroligand [Ge2OS6] and tetrahedral [HgS4] asymmetric functional motifs. This study offers valuable insights into the development of NCS materials through logical chemical substitution strategies, and the intriguing optical properties of this new oxychalcogenide holds promise for applications in the IR-NLO field.