A Ca1.5Ba0.5Si5O3N6:Eu2+ phosphor with a monoclinic lattice in the Cm space group exhibiting a composite structure consisting of CaSi2O2N2-like and BaSi6N8O-like structures was examined in terms of structure and luminescence. The luminescent properties of the Ca1.5Ba0.5Si5O3N6:Eu2+ phosphor could be suitable for light emitting diode applications since it exhibited a promising yellow (or amber) emission peaking at similar to 570-590 nm at excitations of 450-460 nm. The present investigation was focused on verifying the composite structure by employing quantum mechanical calculations such as the Hartree-Fock ab initio calculation and a density functional theory calculation along with precise structural and compositional analyses. The two-peak emission behavior ascribed to the composite structure was also examined in terms of continuous wave time-resolved In addition, the energy and photoluminescence. transfer between two activator sites ascribed to the composite structure was examined in detail.