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

Abstract

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 R3c, 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.