Nd³⁺, Yb³⁺ and Ho³⁺ Codoped Oxyfluoride Glass Ceramics with High Efficient Green Upconversion Luminescence

New oxyfluoride glasses and glass ceramic codoped with Nd³⁺, Yb³⁺ and Ho³⁺ were prepared. The x-ray diffraction analysis revealed that the heat treatments of the oxyfluoride glasses could cause the precipitation of (Nd³⁺, Yb³⁺, Ho³⁺)-doped fluorite-type crystals. Very strong green up-conversion luminescence due to the Ho³⁺: (⁵F₄, ⁵S₂) → ⁵I₈ transition under 800-nm excitation was observed in these transparent glass ceramics. The intensity of the green up-conversion luminescence in a 1-mol% YbF₃-containing glass ceramic was found to be about 120 times stronger than that in the precursor oxyfluoride glass. The reason for the highly efficient Ho³⁺ up-conversion luminescence in the oxyfluoride glass ceramics is discussed.