Structural and spectroscopic insights into the performance of K₃Tb(PO₄)₂ green phosphor

A narrow-band green emitting K₃Tb(PO₄)₂ phosphor has been synthesized by a novel single-crystal growth approach involving the application of an inert flux. Its structural and spectroscopic peculiarities have been systematically studied by means of single-crystal X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy analysis, and diffuse reflectance, infrared and photoluminescence spectroscopy. The layered framework of the title compound is organized by linking together slightly distorted phosphate tetrahedra with non-condensed TbO₇ polyhedra into [Tb(PO₄)₂]^3- sheets interconnected by potassium cations. K₃Tb(PO₄)₂ reveals an intense slow green photoluminescence (PL) (decay time constant τ is near 2.6 ms) under excitation in the range from VUV (100 nm) up to blue light (484 nm). Intensive narrow lines of the ⁵D₄ → ⁷F_6-0 (4f⁸ → 4f⁸) radiative transitions in Tb³⁺ ions form the PL spectra. Besides, fast violet PL (time constant τ < 20 ns) occurs under PL excitations in a narrow range from 250 to 325 nm. Dependencies of the PL quantum yield, and the green and violet PL decays on the excitation wavelength have been analyzed taking into account the multi-phonons, cross-relaxation processes and exciting light absorption by the inherent defects in the K₃Tb(PO₄)₂ crystal lattice. The dependency of the complex characteristics of PL quantum yield on the excitation wavelength and violet photoluminescence is ascribed to the formation of Tb⁴⁺ ions. The color coordinates and color purity of the emission were estimated for the cases of PL excitation in the 160-371 nm spectral range. It has been shown that these characteristics are similar to or even better than those reported for conventional commercial green phosphors. The high values of correlated color temperature (T > 4863 K) of the emission indicates the potential use of K₃Tb(PO₄)₂ for enhancing cold-light emitting devices.