Reduction Mechanisms of Cu2+-Doped Na2O-Al2O3-SiO2 Glasses during Heating in H2 Gas

Controlling valence state of metal ions that are doped in materials has been widely applied for turning optical properties. Even though hydrogen has been proven effective to reduce metal ions because of its strong reducing capability, few comprehensive studies focus on practical applications because of the low diffusion rate of hydrogen in solids and the limited reaction near sample surfaces. Here, we investigated the reactions of hydrogen with Cu²⁺-doped Na₂O-Al₂O₃-SiO₂ glass and found that a completely different reduction from results reported so far occurs, which is dominated by the Al/Na concentration ratio. For Al/Na < 1, Cu²⁺ ions were reduced via hydrogen to metallic Cu, distributing in glass body. For Al/Na > 1, on the other hand, the reduction of Cu²⁺ ions occurred simultaneously with the formation of OH bonds, whereas the reduced Cu metal moved outward and formed a metallic film on glass surface. The NMR and Fourier transform infrared results indicated that the Cu²⁺ ions were surrounded by Al³⁺ ions that formed AlO₄, distorted AlO₄, and AlO₅ units. The diffused H₂ gas reacted with the Al-O^-···Cu⁺ units, forming Al-OH and metallic Cu, the latter of which moved freely toward glass surface and in return enhanced H₂ diffusion.

Temperature dependent red luminescence from a distorted Mn4+ site in CaAl4O7:Mn4+

Thermal luminescence quenching behavior of a phosphor is essential for application in phosphor converted white light emitting diodes (pc-WLEDs) because the phosphor layer can be heated up to 473K in a working high power WLEDs. Here, we have confirmed indeed a red luminescence of Mn(4+) substituting for calcium sites rather than tetrahedral aluminum sites in CaAl(4)O(7):Mn which can be synthesized in pure phase even with boron acid as flux, and examined the low and high temperature luminescent properties in the range of 10 to 500K. We have revealed as well as thermal quenching mechanism that distorted octahedral Mn(4+) sites suffer severe thermal quenching. This work, thus, hints a strategy to find a new Mn(4+) phosphor with better resistance to thermal impact in the future.