Molecular-dynamics modeling of Eu3+-ion clustering in SiO2 glass

This paper reports a molecular-dynamics study on the clustering of Eu3+ ions incorporated in SiO2 glass. Classical molecular-dynamics experiments were carried out on xEu2O3-(100?x)SiO2 (x=0, 1, 2, 3, 4, and 5?mol?%) glasses using empirical pairwise Morse interatomic potentials. Due to statistically improved simulation results, averaged over hundreds of Eu3+ ions, the effects of Eu2O3 content on the silica network, Eu3+ coordination, and on Eu3+ clustering were properly analyzed. At 1?mol?% Eu2O3 most (?70%) of Eu3+ ions are isolated in silica host but have a strained coordination with typically three short bonds to nonbridging oxygens plus one long bond to bridging oxygen. It is found that the Eu3+-O coordination number increases from 4.5 to 4.9 atoms due to an increase in sharing of nonbridging oxygens. This follows an increase in the Eu3+-Eu3+ coordination number due to an increasing clustering of Eu3+ ions. Eu3+ clustering is already present in the sample with 1?mol?% Eu2O3, but most of the clustered Eu3+ ions are present only in pairs. As Eu2O3 content increases in the glass, both the proportion of Eu3+ ions involved in clusters and the size of individual clusters increase, and signs of phase separation become apparent for 5?mol?% Eu2O3.