Homovalent cation substitutions leading to new lithium-containing La3CuSiS7 family members having chiral, uniaxial structures and exhibiting second- and third-harmonic generation.

Eighteen isotypic, lithium-containing rare-earth sulfides, crystallizing in the chiral, polar, and noncentrosymmetric space group P 6 3 with the La 3 CuSiS 7 -structure type have been prepared via direct combination of the elements or binary sulfides; nine of these compounds are reported for the first time. The structures of these compounds, with the formulae Ln 3 LiTS 7 (Ln = La, Ce, Pr, Nd, Sm, Gd, and Dy for T = Si or Ge; Ln = La, Ce, Pr, and Nd for T = Sn), were determined by single crystal X-ray diffraction. According to X-ray powder diffraction data, the Ln 3 LiTS 7 compounds are the major phase of the reaction products. All of the compounds are semiconductors with optical bandgaps spanning nearly the entire visible region. The Si- and Ge-containing analogs show high thermal stability,> 1000 °C, while the Sn-containing compounds melt in the vicinity of ~740 °C. The compounds show potentially broad regions of optical transparency in the infrared regime. Calculated bond valence sums (BVSs) and global instability index (G) values confirm the Ln3+ oxidation state and stable crystal structures with reasonable strain, respectively. Ce 3 LiGeS 7 and Ce 3 LiSnS 7 display non-phase-matching but significant second-harmonic generation responses with χ 2 values of 21 ± 1 and 28 ± 1 pm/V, respectively at λ = 1.8 µm. The laser-induced damage threshold values for Ce 3 LiGeS 7 and Ce 3 LiSnS 7 are> 3 × commercial AgGaSe 2 , for picosecond pulses at λ = 1.064 µm. These two compounds also exhibit moderate third-harmonic generation responses. Additionally, crystal-chemical correlations are discussed. [Display omitted] • Nine new members of the La 3 CuSiS 7 family have been discovered, e.g. Ce 3 LiSiS 7. • Ce 3 LiGeS 7 and Ce 3 LiSnS 7 show second-harmonic generation with χ 2 values of> 20 pm/V. • The laser-induced damage thresholds for Ce 3 LiGeS 7 and Ce 3 LiSnS 7 are ~3 × AgGaSe 2. • Ce 3 LiGeS 7 and Ce 3 LiSnS 7 display moderate third-harmonic generation responses. [ABSTRACT FROM AUTHOR]