Tunable Broadband Visible Emission Achieved by Phase Transformation-Driven Self-Reduction of Eu3+ to Eu2+ in Calcium Phosphate Matrix

Abstract

In this work, we report the synthesis of Eu2+-doped alpha-tricalcium phosphate (α-TCP, α-Ca3(PO4)2) via a phase transformation of Eu3+-doped CaHPO4 ∙ 2H2O. The phase conversion accompanied by a reduction of Eu3+ to Eu2+ occurred during the annealing of the starting material in a vacuum. The optical properties of the obtained α-TCP:Eu2+ were investigated by photoluminescence (PL), thermally stimulated luminescence, and persistent luminescence decay measurements. The obtained material exhibited tunable broadband PL with FWHM values ranging from 94 to 142 nm at room temperature. The PL can be tuned in terms of emission maximum and FWHM by varying the excitation wavelength. The broadband emission was achieved due to the multi-site occurrence of Eu2+ ions in the complex α-TCP matrix. Three types of traps were determined with activation energy values of 0.80, 0.75, and 0.68 eV. After irradiation with X-rays, an afterglow characterizable by a Eu2+ broadband spectrum with a maximum at around 480 nm can be detected for at least 10 h.