Discovery of New Solid Solution Phosphors via Cation Substitution-Dependent Phase Transition in M₃(PO₄)₂:Eu²⁺ (M = Ca/Sr/Ba) Quasi-Binary Sets

The cation substitution-dependent phase transition was used as a strategy to discover new solid solution phosphors and to efficiently tune the luminescence property of divalent europium (Eu²⁺) in the M₃(PO₄)₂:Eu²⁺ (M = Ca/Sr/Ba) quasi-binary sets. Several new phosphors including the greenish-white SrCa₂(PO₄)₂:Eu²⁺, the yellow Sr₂Ca­(PO₄)₂:Eu²⁺, and the cyan Ba₂Ca­(PO₄)₂:Eu²⁺ were reported, and the drastic red shift of the emission toward the phase transition point was discussed. Different behavior of luminescence evolution in response to structural variation was verified among the three M₃(PO₄)₂:Eu²⁺ joins. Sr₃(PO₄)₂ and Ba₃(PO₄)₂ form a continuous isostructural solid solution set in which Eu²⁺ exhibits a similar symmetric narrow-band blue emission centered at 416 nm, whereas Sr²⁺ substituting Ca²⁺ in Ca₃(PO₄)₂ induces a composition-dependent phase transition and the peaking emission gets red shifted to 527 nm approaching the phase transition point. In the Ca_3–<i>x</i>Ba_<i>x</i>(PO₄)₂:Eu²⁺ set, the validity of crystallochemical design of phosphor between the phase transition boundary was further verified. This cation substitution strategy may assist in developing new phosphors with controllably tuned optical properties based on the phase transition