A solid-solution-phase Ba_1.75Ca_1.25MgSi₂O₈: Eu²⁺, Mn²⁺ phosphor in the photosynthetic action spectrum with dual band emissions at 438 nm and 660 nm is fabricated. X-ray diffraction (XRD) confirms the presence of the solid-solution phase. With the supporting information from the diffuse reflection spectrum, a feasible way to obtain higher energy-transfer (ET) efficiency is attained, and the ET efficiency of Eu²⁺-Mn²⁺ is enhanced to 76%. The mechanism of this enhancement is owing to variation of the solid solution composition of Ca₃MgSi₂O₈ and Ba₃MgSi₂O₈, which contributes to the extension of the critical distance. Temperature-dependent results show an enhancement which is attributed to Ca. These enhancements show great promise for improving eco-lighting devices.