Luminescence, cathodoluminescence and Ce3+ → Eu2+ energy transfer and emission enhancement in the Sr5(PO4)3Cl:Ce3+,Eu2+ phosphor

Abstract

Ultraviolet and vacuum ultraviolet spectra are reported for Ce³⁺ and Eu²⁺ singly doped into the Sr₅(PO₄)₃Cl lattice, synthesized by a solid-state process. The maxima of the emission intensities are at 346 nm (Ce³⁺) and 442 nm (Eu²⁺), with lifetimes of 25 ns and 494 ns, respectively. The Ce³⁺ excitation spectra exhibit five bands assigned to vibronic structures of the 4f¹ → 5d¹ electronic transition, in addition to the host absorption at 166 nm at 295 K. The 5d centroid shift is similar to that in SrCl₂:Ce³⁺. The Eu²⁺ excitation spectrum is mainly comprised of two 4f⁷ → 4f⁶5d broad bands between 200 nm and 450 nm, peaking at 343 nm and 275 nm. Both of the singly doped systems exhibit concentration quenching, with energy transfer rates being in the low (μs)⁻¹ range for concentrations up to 4 at.% of total cations. The energy transfer rates are linearly and quadratically related to dopant ion concentrations of Ce³⁺ and Eu²⁺, respectively. The energy transfer between Ce³⁺ and Eu²⁺ in the co-doped Sr₅(PO₄)₃Cl lattice has been studied by the analysis of intensity and decay measurements of both Ce³⁺ and Eu²⁺ emissions. The electric dipole–electric dipole transfer has an efficiency of 91% in Sr_4.8−xCe_0.01Eu_xNa_0.01(PO₄)₃Cl, with a critical distance of 21 Å. Although energy transfer between Ce³⁺ and Eu²⁺ upon excitation into the overlapping Ce³⁺/Eu²⁺ absorption band is definitely demonstrated by the dramatic shortening of Ce³⁺ lifetime, the emission of Eu²⁺ is also enhanced at low Ce³⁺ concentrations using exclusive excitation into the Eu²⁺ absorption band. The quantum yields (QY) of the co-doped system approach those of BaMgAl₁₀O₁₇:Eu²⁺ (BAM) for the excitation wavelengths of 317 and 365 nm, but are inferior for 254 nm excitation. The addition of 1 at.% Ce³⁺ to Sr_4.98Eu_0.02(PO₄)₃Cl increases the QY by ∼20% of the original value. The cathodoluminesce of the co-doped phosphor is comparable to that of BAM, with CIE coordinates of the emission (0.274, 0.237), so that the application as a field emission display phosphor is proposed.