Structural, Optical, and Electronic Properties of Two Quaternary Chalcogenide Semiconductors: Ag₂SrSiS₄ and Ag₂SrGeS₄

Abstract

materials have shown particular versatility and promise among these compounds. These semiconductors take advantage of a diverse bonding scheme and chemical differences among cations to target a degree of antisite defect resistance. Within this set of compounds, the materials containing both Ag and Sr have not been experimentally studied and leave a gap in the full understanding of the family. Here, we have synthesized powders and single crystals of two Ag-and Sr-containing compounds, Ag 2 SrSiS 4 and Ag 2 SrGeS 4 , each found to form in the tetragonal I4̅ 2m structure of Ag 2 BaGeS 4 . During the synthesis targeting the title compounds, two additional materials, Ag 2 Sr 3 Si 2 S 8 and Ag 2 Sr 3 Ge 2 S 8 , have also been identified. These cubic compounds represent impurity phases during the synthesis of Ag 2 SrSiS 4 and Ag 2 SrGeS 4 . We show through hybrid density functional theory calculations that Ag 2 SrSiS 4 and Ag 2 SrGeS 4 have highly dispersive band-edge states and indirect band gaps, experimentally measured as 2.08(1) and 1.73(2) eV, respectively. Second-harmonic generation measurements on Ag 2 SrSiS 4 and Ag 2 SrGeS 4 powders show frequencydoubling capabilities in the near-infrared range.

Cubic Crystal Structure Formation and Optical Properties within the Ag–BII–MIV–X (BII = Sr, Pb; MIV = Si, Ge, Sn; X = S, Se) Family of Semiconductors

“…Expanding from the Ag–Pb–Si–S and Ag–Sr–Sn–S systems, we also considered three additional isostructural materials: Ag 2 Sr 3 Si 2 S 8 , Ag 2 Sr 3 Ge 2 S 8 , and Ag 2 Sr 3 Ge 2 Se 8 . The single-crystal structures of each of these three additional compounds have been previously reported as Ag 2 Sr 3 Si 2 S 8 , Ag 4 Sr 6 Ge 4 S 16 , and Ag 2 Sr 3 Ge 2 Se 8 , but given that the three materials share the same structure as Ag 2 Pb 3 Si 2 S 8 and Ag 2 Sr 3 Sn 2 S 8 (detailed in the experimental results below), the computational geometry of Ag 2 Sr 3 Sn 2 S 8 (this work) is used to build the input structures for DFT-HSE06 calculations concerning these materials. The Brillouin zone and k path used for the calculations of the cubic structures are shown in Figure S3, and the Γ-point-centered k -point grids used for the DFT-HSE06 calculations and respective input lattice parameters are listed in Table S12.…”

“…Alongside the A I 2 –B II –M IV –X 4 -type compounds is a related family of cubic materials that share the same grouping of elements, including compounds such as (Ag,Li) 0.5 Pb 1.75 GeS 4 , Cu 2 Ba 3 Sn 2 S 8 , Ag 4 Sr 6 (Ge,Sn) 4 (S,Se) 16 , , and Ag 2 Sr 3 Si 2 S 8 . These cubic semiconductors have been shown to have large second-harmonic generation responses and the potential for NLO applications that do not require phase-matching. , Within these two groups of semiconductorsi.e., the A I 2 –B II –M IV –X 4 and cubic A I –B II –M IV –X materialsthe Ag–Pb–Ge–S phase system is of particular note because it can readily exist in both groups.…”

“…However, the combinatorial freedom associated with assembling complex crystal structures of the quaternary materials poses a significant challenge to exploring this materials space reliably. While the structural and phase relationships specifically among the A I 2 –B II –M IV –X 4 -type compounds have been previously considered, , there has been relatively less attention focused on the cubic structural types and the impact of stoichiometry on the formation more generally of A I –B II –M IV –X materials.…”

“…However, only the Ag 2 Pb 3 Si 2 S 8 and Ag 2 Sr 3 Sn 2 S 8 stoichiometries appear to form single-phase materials. High-quality single crystals of both Ag 2 Pb 3 Si 2 S 8 and Ag 2 Sr 3 Sn 2 S 8 were synthesized and examined by single-crystal XRD, indicating that each compound adopts the noncentrosymmetric cubic I 4̅3 d structure type (isostructural to Ag 2 Sr 3 Ge 2 Se 8 and Ag 2 Sr 3 Si 2 S 8 ). , The I 4̅3 d structure assignment holds at both room temperature and 80 K, with only a slight temperature-dependent lattice constant reduction and small changes to the occupation of Ag lattice sites in the structures being noted. At 80 K, refinement of the Ag lattice site occupational disorder in Ag 2 Pb 3 Si 2 S 8 and Ag 2 Sr 3 Sn 2 S 8 aligns with the proposed model for Ag 4 Sr 6 Sn 4 S 16 .…”

Binary Cu2–xS Templates Direct the Formation of Quaternary Cu2ZnSnS4 (Kesterite, Wurtzite) Nanocrystals

“…While several synthetic routes have achieved high-quality CZTS NCs, , the conventional approach involves the injection of a sulfur precursor into a heated solution containing the metal precursors. This hot injection strategy has been employed in the synthesis of many binary chalcogenide NCs, − but encounters additional complexity when applied to quaternary materials where the larger combinatorial space can include undesired phases and stoichiometries. , At the core of the synthetic challenge is the presence of multiple metal saltseach with differing solubilities and reactivities toward the sulfur precursor and ligandsthat complicate the nucleation of a uniform phase. In fact, several undesired multinary intermediate species have been observed in the one-pot synthesis of CZTS NCs, including Cu 2‑x S, SnS, ZnS, and ternary (Cu 2 SnS 3 ), where each can introduce defects in an operating device. − A further mechanistic challenge in this synthesis is the possible transient formation of metallic Cu nanoparticles in situ .…”