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 P6(3) with the La3CuSiS7-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 La3CuSiS7 (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 La3CuSiS7 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 degrees C, while the Sn-containing compounds melt in the vicinity of similar to 740 degrees 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 Ln(3+) oxidation state and stable crystal structures with reasonable strain, respectively. Ce3LiGeS7 and Ce3LiSnS7 display non-phase-matching but significant second-harmonic generation responses with chi((2)) values of 21 +/- 1 and 28 +/- 1 pm/V, respectively at lambda = 1.8 mu m. The laser-induced damage threshold values for Ce3LiGeS7 and Ce3LiSnS7 are > 3 x commercial AgGaSe2, for picosecond pulses at lambda = 1.064 mu m. These two compounds also exhibit moderate third-harmonic generation responses. Additionally, crystal-chemical correlations are discussed. (C) 2022 Published by Elsevier B. V.

Automated summary

Eighteen isotypic, lithium-containing rare-earth sulfides with the La3CuSiS7-structure type have been synthesized and characterized. These compounds are semiconductors with wide optical bandgaps in the visible region. They exhibit high thermal stability and potentially broad regions of optical transparency in the infrared regime. Two of these compounds, Ce3LiGeS7 and Ce3LiSnS7, show significant second-harmonic generation responses and high laser-induced damage threshold values.