Synthesis and luminescence properties of single-phase Ca₂P₂O₇:Eu²⁺, Eu³⁺ phosphor with tunable red/blue emission

A series of Eu²⁺ and Eu³⁺ coexisting Ca₂P₂O₇ phosphors with tunable red/blue (R/B) emission have been successfully synthesized by a two-step method. Phase composition, luminescence properties and surface states are investigated by the X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) emission spectra, fluorescence quantum efficiency, fluorescent lifetime and X-ray photoelectron spectroscopy (XPS). It can be concluded from XRD and FTIR results that pure Ca₂P₂O₇ phase can be obtained at 900 °C and Ca₃(PO₄)₂ impurity phase occurs at an increasing preparation temperature (1100 °C). The XRD results confirm that the Eu²⁺or Eu³⁺ doped samples all exhibit pure tetragonal Ca₂P₂O₇ crystal with space group P4₁. Under excitation of 330 nm, Eu²⁺ singly-doped Ca₂P₂O₇ phosphors showed a broad blue emission band at 422 nm, while the Eu³⁺ single-doped samples emitted a strong orange-red light with wavelength of 591, 597 and 618 nm excited by 395 nm. In particular, oxygen vacancies in pure Ca₂P₂O₇ crystal are found due to the strong PL emission at 420 nm. Furthermore, Eu²⁺/Eu³⁺ co-doped Ca₂P₂O₇ phosphors prepared by this two-step synthesis show the blue emission at 422 nm (Eu²⁺ ions) and the orange-red emission (Eu²⁺ ions) at 591 nm, 597 nm and 618 nm under near-ultraviolet (NUV) excitation of 380 nm. It was detected that the quantum efficiency of the Ca_1.88P₂O₇:0.12Eu phosphor was 37.7%. The tunable R/B emission ratio from 0.13 to 1.56 can be achieved by controlling the recalcined temperature in the second step when excited by 380 nm. The XPS demonstrate the existence of Eu³⁺ ions on the surface of Eu²⁺/Eu³⁺ co-doped Ca₂P₂O₇ phosphors. Under NUV excitation in the range from 350 to 400 nm, the optimum Commission International de I’Eclairage (CIE) coordinate of Ca₂P₂O₇:0.12Eu²⁺/Eu³⁺ phosphors is (0.3648, 0.215), which can be potentially used as an emitting source to meet the needs of illumination devices.