The Nitrogen-Hole-Center Electron Transfer Imparts Reduction Ability to Eu Ion in AlN-Containing Phosphate Glasses

Abstract

The europium ion possesses fascinating luminescent properties, and its oxidation state is largely governed by the interaction with doping matrixes due to the appropriate redox potential. Here, we unravel the connection between the phosphate oxynitride glass structure and stepwise valence changes from Eu3+ to Eu2+. Through tuning the introducing content of AlN in phosphate glasses, an effective reduction among Eu3+ ions could be readily realized. This reduction process is explained by the “Nitrogen-Hole-Center (NHC) electron transfer” model. In this model, NHC and Eu3+ ions play as electron donor and acceptor roles, respectively. Under continuous thermal vibration of the lattice, the electrons released from unstable NHC could be caught by their nearest neighbor Eu3+ ions. Moreover, nitrogen-to-oxygen substitution significantly enhances the covalency and rigidity of the glassy network by forming distorted [PO3N] and [PO2N2] tetrahedrons, creating a perfect shield for Eu2+ and resisting the attack of oxygen.