A Cab>1.5b>Bab>0.5b>Sib>5b>Ob>3b>Nb>6b>:Eu2+ phosphor with a monoclinic lattice in the Cm space group exhibiting a composite structure consisting of CaSib>2b>Ob>2b>Nb>2b>-like and BaSib>6b>Nb>8b>O-like structures was examined in terms of structure and luminescence. The luminescent properties of the Cab>1.5b>Bab>0.5b>Sib>5b>Ob>3b>Nb>6b>:Eu2+ phosphor could be suitable for light-emitting diode applications since it exhibited a promising yellow (or amber) emission peaking at ∼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 and time-resolved photoluminescence. In addition, the energy transfer between two activator sites ascribed to the composite structure was examined in detail.