Tunable broadband visible emission achieved by phase transformation-driven self-reduction of Eu³⁺ to Eu²⁺ in a calcium phosphate matrix

In this work, we report the synthesis of Eu²⁺-doped alpha-tricalcium phosphate (α-TCP, α-Ca₃(PO₄)₂) <i>via</i> 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 87 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 an Eu²⁺ broadband spectrum with a maximum at around 480 nm can be detected for at least 10 h.