Two isostructural ternary acentric sulfides, Y6Si2.5S14 (1) and Y6Ge2.5S14 (2) were re-investigated to understand the origin of the chemical flexibility of RE6BxCyCh14(RE=Y, La−Lu; B=Si, Ge, Sn, Al, Ga; C=monovalent M+ (Ag, Na, Li, etc.), divalent M2+ (Mg, Cr, Ni, Zn, etc.), trivalent M3+ (Al, In, Ga, etc.), tetravalent M4+ (Si, Ge, Sn) and pentavalent M5+ (Sb), Ch=S, Se), which consists of ∼444 isostructural compounds. Y6IV2.5S14 (IV=Si, Ge) were synthesized by a high-temperature salt flux method. The crystal structures of Y6IV2.5S14 (IV=Si, Ge) are constructed by [YS8] polyhedra, [Si1S6] octahedra, and [Si2S4] tetrahedra. The Si1 atom displaces from the center of [Si1S6] octahedra with partial occupancy, which can be replaced by various metals, and mainly accounts for the chemical flexibility of the RE6BxCyCh14 family. The bonding pictures of Y6Si2.5S14 were studied by electron localization function (ELF) and crystal orbital Hamilton population (COHP) calculations. Y6Si2.5S14 is evaluated as an indirect semiconductor with a bandgap of 2.4(1) eV measured by UV-Vis. The indirect bandgap of Y6Ge2.5S14 is 1.7(1) eV. Y6IV2.5S14 (IV=Si, Ge) are not type-I phase-matchable materials. For samples of 47 μm particle size, Y6Si2.5S14 and Y6Ge2.5S14 own good second harmonic generation (SHG) responses of ∼3.0×AGS and ∼2.8×AGS respectively. Y6Si2.5S14 and Y6Ge2.5S14 possess high laser damage threshold (LDT) of ∼5.5×AGS and ∼5.2×AGS respectively.