[Sr4Cl2][Si3S9]: Ultrawide-bandgap salt-inclusion thiosilicate nonlinear optical material with unprecedented tri-polymerized [Si3S9] clusters

Abstract

Adjusting the inherent incompatibility between a wide optical bandgap (E_g) and strong second-harmonic-generation (SHG) response (d_ij) has become an urgent challenge for the design and exploration of outstanding infrared nonlinear optical (IR NLO) materials. To address this, a new salt-inclusion thiosilicate, [Sr₄Cl₂][Si₃S₉], exhibiting the first discovered tri-polymerized [Si₃S₉] clusters, was successfully designed and synthesized by combining the native performance advantages of halides and sulfides. Detailed experimental and theoretical investigations showed that the uniformly aligned [Si₃S₉] functional groups could make a great contribution to the large phase-matching (PM) d_ij (1.2 × AgGaS₂), while the ionic salt enabled a positive blue-shift of the absorption edge to achieve the widest E_g (4.22 eV) reported among all known NLO chalcohalides. Furthermore, [Sr₄Cl₂][Si₃S₉] is the first example showing a superior balance between E_g > 4.0 eV and d_ij > 1.0 × AgGaS₂ in the PM chalcohalide system and could thus effectively eliminate the undesirable property incompatibility, indicating that salt-inclusion thiosilicates are potential research candidates for exploring ultrawide-bandgap IR SHG materials.