Mineral apatite compounds have attracted sig-nificant interest due to their chemical stability and adjustable hexagonal structure,which makes them suit-able as new photovoltaic functional materials.The band gap of natural apatite is ～ 5.45 eV,and such a large value limits their applications in the field of catalysis and energy devices.In this research,we designed a method to narrow the band gap via the tetrahedral substitution effect in apatite-based compounds.The density functional theory(DFT) and experimental investigation of the electronic and optical properties revealed that the continuous incorpora-tion of[MO4]4-tetrahedrons (M =Si,Ge,Sn,and Mn) into the crystal lattice can significantly reduce the band gap.In particular,this phenomenon was observed when the[MnO4]4-tetrahedron replaces the[PO4]4-tetrahedron because of the formation of a Mn 3d-derived conduction band minimum (CBM) and interacts with other elements,leading to band broadening and obvious reduction of the band gap.This approach allowed us to propose a novel scheme in the band gap engineering of apatite-based compounds toward an entire spectral range modification.