Phase Transformation in Ca3(PO4)2:Eu2+ via the Controlled Quenching and Increased Eu2+ Content: Identification of New Cyan-Emitting α-Ca3(PO4)2:Eu2+ Phosphor

A case of phosphor is reported where the cooling rate parameter significantly influences the luminescence property. By quenching the sample after the high-temperature solid-state reaction at 1250°C, we successfully prepared the Eu²⁺-doped α form Ca₃( PO₄)₂ (α- TCP:Eu²⁺) as a new kind of bright cyan-emitting phosphor. The unusual emission color variation (from cyan to blue) depends on the cooling rate after sintering and Eu²⁺ doping level as it was observed in the TCP-based phosphors. By the Rietveld analysis, it is revealed that the cyan- and blue-emitting phosphors are two different TCP forms crystallizing in the monoclinic (space group P2₁/ a, α- TCP) and the rhombohedral structure (space group R3 c, β- TCP), respectively. Upon 365 nm UV light excitation, α- TCP:Eu²⁺ exhibits an asymmetric broad-band cyan emission peaking at 480 nm, while β- TCP:Eu²⁺ displays a relatively narrow-band blue emission peaking at 416 nm. The Eu²⁺-doping in Ca₃( PO₄)₂ shifts the upper temperature limit of the stable structural range of β form from 1125°C to ≥1250°C. Moreover, the crystal structures of α/β- TCP:Eu²⁺ were compared in the aspects of compactness and cation site sets. The emission thermal stability of α/β- TCP:Eu²⁺ was comparatively characterized and the difference was related to the specific host structural features. [ABSTRACT FROM AUTHOR]