Extended X-ray Absorption Fine Structure (EXAFS) Analysis of Vitreous Rare Earth Sodium Phosphates

The local structure around rare-earth ions (RE$^{3+})$ in rare-earth ultraphosphate (REUP) glasses has been studied using RE L$_{III}$ edge (RE $=$ Nd, Er, Dy, and Eu) and K edge (RE $=$ Nd, Pr, Dy, and Eu) extended X-ray absorption fine structure (EXAFS) spectroscopy. (RE$_{2}$O$_{3})_{x}$(Na$_{2}$O)$_{y}$(P$_{2}$O$_{5})_{1-x-y}$ glasses in the compositional range 0 $\le $ x $\le $ 0.14 and x $+$ y $=$ 0.3 and 0.4 were studied. RE-oxygen (RE-O) coordination number decreases from $\sim$ 10 to $\sim$ 7.5 with increasing RE-content for Nd, Pr, Eu, and Dy. For Er, RE-O coordination number increases from $\sim$ 8.7 to $\sim$ 10 with increasing RE-content. For the first oxygen shell, the RE-O distance ranges between 2.41-2.43 {\AA}, 2.44-2.46 {\AA}, 2.24-2.26 {\AA}, 2.28-2.32 {\AA}, and 2.32-2.36 {\AA} for Nd, Pr, Er, Dy, and EU glasses, respectively. Second shell around RE ions consists of phosphorus atoms, with RE-P distance about 3.0-3.5 {\AA} and coordination number ranging from 1 to 3. The third shell primarily contains oxygen and is at a distance about 4.0-4.1 {\AA} from RE ions.