Enhanced green emission of Eu²⁺ by energy transfer from the ⁵D₃ Level of Tb³⁺ in NaCaPO₄

Eu²⁺/Tb³⁺ single-doped and codoped NaCaPO₄ phosphors were prepared by a high-temperature solid-state reaction method. Their VUV-UV excitation spectra, emission spectra, fluorescence decay spectra, X-ray excited luminescence (XEL), and cathodoluminescence (CL) spectra were investigated and discussed. The main parameters of the electron-vibrational coupling between the Eu²⁺ 5d states and host lattice vibrations (such as Huang-Rhys factor, effective phonon energy, and position of the zero-phonon line of the 4f⁶5d-4f⁷ emission of Eu²⁺) were estimated from the experimental spectra. For codoped samples NaCaPO ₄:Eu²⁺, Tb³⁺, enhanced green emission of Eu²⁺ by energy transfer (ET) from the ⁵D₃ level of Tb³⁺ was found.