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

Abstract

Eighteen isotypic, lithium-containing rare-earth sulfides, crystallizing in the chiral, polar, and noncentrosymmetric space group P63 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 Ln3LiTS7 (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 Ln3LiTS7 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. Ce3LiGeS7 and Ce3LiSnS7 display non-phase-matching but significant second-harmonic generation responses with χ2 values of 21 ± 1 and 28 ± 1 pm/V, respectively at λ = 1.8 µm. The laser-induced damage threshold values for Ce3LiGeS7 and Ce3LiSnS7 are> 3 × commercial AgGaSe2, for picosecond pulses at λ = 1.064 µm. These two compounds also exhibit moderate third-harmonic generation responses. Additionally, crystal-chemical correlations are discussed.