Reduction Mechanism of Eu3+ in Ca2+xLa8−x(SiO4)6O2–0.5x: Eu2+ Phosphors

Abstract

A series of apatite phosphors with a general formula of Ca2+xLa8−x(SiO4)6O2–0.5x: Eu2+ were successfully synthesized by a conventional solid-state method. The increasing substitution of Ca2+ for La3+ ions in the crystal favors the reduction from Eu3+ to Eu2+ ions, resulting in phosphors with a strong green emission at 510 nm. XANES results reveal that when the value of x increases, the amount of Eu2+ also increases. Rietveld refinement shows that the produced materials belong to the hexagonal crystal system, with a space group of P6/3 m (176). Based on the Rietveld refinement, EPR, and XANES results (from Rietveld refinement, EPR, and XANES analyses), it is suggested that the non-equivalent (i.e. lowering of the valence from +3 to +2) substitution of Ca2+ for La3+ leads to a decrease of the number of O2− ions in the crystal and, thus, favors the reduction of Eu3+ to Eu2+.