Two isostructural ternary acentric sulfides, Y 6 Si 2.5 S 14 ( 1 ) and Y 6 Ge 2.5 S 14 ( 2 ) were re-investigated to understand the origin of chemical flexibility of RE 6 B x C y Ch 14 (RE=Y, La-Lu; B=Si, Ge, Sn, Al, Ga; C= monovalent M + (Ag, Na, Li, etc.), divalent M 2+ (Mg, Cr, Ni, Zn, etc.), trivalent M 3+ (Al, In, Ga, etc.), tetravalent M 4+ (Si, Ge, Sn) and pentavalent M 5+ (Sb), Ch=S, Se), which consists of ~444 isostructural compounds. Y 6 IV 2.5 S 14 (IV=Si, Ge) were synthesized by a high-temperature salt flux method. The crystal structures of Y 6 IV 2.5 S 14 (IV=Si, Ge) are constructed by [YS 8 ] polyhedra, [Si1S 6 ] octahedra, and [Si2S 4 ] tetrahedra. The Si1 atom displaces from the center of [Si1S 6 ] octahedra with partial occupancy, which can be replaced by various metals, and mainly accounts for the chemical flexibility of the RE 6 B x C y Ch 14 family. The bonding pictures of Y 6 Si 2.5 S 14 were studied by electron localization function (ELF) and crystal orbital Hamilton population (COHP) calculations. Y 6 Si 2.5 S 14 is evaluated as an indirect semiconductor with a bandgap of 2.4(1) eV measured by UV-Vis. The indirect bandgap of Y 6 Ge 2.5 S 14 is 1.7(1) eV. Y 6 IV 2.5 S 14 (IV= Si, Ge) are not type-I phase-matchable materials. For samples of 58 µm particle size, Y 6 Si 2.5 S 14 and Y 6 Ge 2.5 S 14 own good SHGn responses of ~ 3.0×AGS and ~2.8×AGS respectively.