Rational combination of multiple structural groups on regulating nonlinear optical property in hexagonal Ln3MGeS7 polar crystals

Combination of multiple structural groups was developed as one effective method for regulating material property and exploring new nonlinear optical (NLO) crystals. For that reason, four Ln ₃MGeS ₇ (Y ₃LiGeS ₇, Y ₃Cd _0.5GeS ₇, Gd ₃Cd _0.5GeS ₇ and Sm ₃Zn _0.5GeS ₇) crystals with various structural motifs were successfully synthesized and systematically investigated. Note that Y ₃LiGeS ₇ and Y ₃Cd _0.5GeS ₇ were firstly synthesized so far. Performance test shows that Ln ₃MGeS ₇ compounds exhibit the variable optical bandgaps (1.62–2.33 eV) and good second harmonic generation (SHG) responses (0.7–1.1 × commercial AgGaS ₂) with requisite phase-matching behavior, which agree well with the theoretical results (Y ₃LiGeS ₇: Δ n = 0.058 and d ₃₃ = 19.97 pm/V; Y ₃Cd _0.5GeS ₇: Δ n = 0.055 and d ₃₃ = 20.6 pm/V) and introduction of M ^IIB (Zn, Cd) atoms into crystal structure is conducive to enlarge the SHG effect. SHG density and dipole-moment calculations indicate that their SHG effects are originated from the major contribution on LnS _n units with moderate effect of GeS ₄ and M ^IIBS ₆ (Zn, Cd) ligands. Therefore, selecting appropriate functional motifs (LnS _n and M ^IIB ) into crystal structure can promote the enhancement of NLO property and further realize the rational design of new promising infrared NLO materials.