Craig, Andrew J.
[Author];
Cho, Jeong Bin
[Author];
Shin, Seung Han
[Author];
Ha, Seung Heon
[Author];
Stoyko, Stanislav S.
[Author];
Jang, Joon I.
[Author];
Aitken, Jennifer Ann
[Author]
Homovalent Cation Substitutions Leading to New Lithium-Containing La3cusis7 Family Members Having Chiral, Uniaxial Structures and Exhibiting Second- and Third-Harmonic Generation
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Title:
Homovalent Cation Substitutions Leading to New Lithium-Containing La3cusis7 Family Members Having Chiral, Uniaxial Structures and Exhibiting Second- and Third-Harmonic Generation
Description:
Eighteen isotypic, quaternary, 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 at temperatures ranging from 850-1100 °C; 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. The structure contains bicapped trigonal prisms of LnS8 for the majority of the compounds and monocapped trigonal prisms for the others. The lanthanide-sulfur polyhedra, TS4 tetrahedra and distorted LiS6 octahedra all align in one crystallographic direction, contributing to the noncentrosymmetric structure. The bicapped trigonal prisms share faces with one-another to form a 3-D network, join with the TS4 tetrahedra via corner and edge sharing, and create tunnels along the c-axis where the lithium ions reside. According to X-ray powder diffraction data, the Ln3LiTS7 compounds are the major phase of the reaction products. Diffuse reflectance spectra indicate that all of the compounds are semiconductors with optical bandgaps spanning nearly the entire visible region. Differential thermal analysis shows relatively high thermal stability for the Si- and Ge-containing analogs up to 1000 °C, while the Sn-containing compounds melt congruently in the vicinity of ~740 °C. Attenuated total reflectance measurements reveal 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 nonphase-matching but significant second-harmonic generation responses with χ(2) values of 21±1 and 28±1 pm/V, respectively at λ=1.8 mm. The laser-induced damage threshold values for Ce3LiGeS7 and Ce3LiSnS7 are >3× commercial AgGaSe2 with values of of 1.27 and 1.26 GW/cm2, respectively, for picosecond pluses at λ=1.064 mm. These two compounds also exhibit good third-harmonic generation responses with χ(3) values ~3× AgGaSe2. Additionally, crystal-chemical correlations are discussed