A novel Mn(II)-based green phosphor and its self-reduction mechanism

A novel Mn²⁺ doped Li₂CdSiO₄ phosphors were synthesized in 1100 °C without reducing atmosphere. Their structure, self-reduction mechanism and luminescence properties were evaluated. Oxygen vacancies were confirmed to exist in Li₂CdSiO₄ by electron spin-resonance spectroscopy and bond volume polarizability of Li₂CdSiO₄ was calculated to prove the lattice oxygen (O3) may easily form oxygen defect ($V_o^{•}$). After different valence Mn source (MnO₂ and MnCO₃) as raw materials were built into Li₂CdSiO₄, similar emission bands with a maximal value of 515 nm appeared under 226 nm excitation and it was ascribed that energy transfer occurred from host to Mn²⁺ ions. Meanwhile, it was demonstrated that a series of redox reactions does happen from MnCO₃ to Mn₃O₄ with increasing calcination temperature. Eventually, Mn₃⁺ (Mn₃O₄) ions were reduced to Mn²⁺ through electron transfer from $V_o^{•}$ to Mn³⁺. Furthermore, interstitials oxygen (O_i) was detected in Li₂CdSiO₄:0.02Mn sample through X-ray photoelectron spectroscopy (XPS) and the special tunnel structure may provide a suitable position to stabilize O_i in air. Direct spectroscopic evidence and the assumed mechanism have been presented for explaining these processes. This research provides a new perspective for defects induced Mn²⁺ emission in designing broad band green phosphors. More importantly, the present work further deepens the understanding of the self-reduction mechanism of Mn²⁺.