EXAFS studies of rare-earth metaphosphate glasses

Abstract

An extended x-ray-absorption fine structure (EXAFS) study has been carried out on a range of rare-earth metaphosphate $R{(\mathrm{P}{\mathrm{O}}_{3})}_{3}$ glasses of growing interest in optical communications and laser technologies. Phosphate glasses modified using the rare-earth oxides ${\mathrm{Pr}}_{6}$${\mathrm{O}}_{11}$, ${\mathrm{Nd}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{Eu}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{Gd}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{Tb}}_{2}$${\mathrm{O}}_{3}$, and ${\mathrm{Ho}}_{2}$${\mathrm{O}}_{3}$, have been investigated using their respective rare-earth ${L}_{\mathrm{III}}$ absorption edges. The data provide information on the local environment of the rare-earth ion within the phosphate glass matrix constructed from linked P${\mathrm{O}}_{4}$ tetrahedra. The rare-earth ions occupy sites with an average coordination number in the range, $6<~N<~8$, the surrounding atoms being oxygen. The first shell interatomic distance over the range of rare-earth ions establishes the rare-earth contraction of ionic radii with increasing atomic number in a series of glasses. There is also evidence for a rare-earth-phosphorus correlation between 2.7 and 3.6 \AA{}, and a further rare-earth-oxygen correlation at approximately 4 \AA{}. The EXAFS spectrum shows no evidence for R-R correlations within the short-range order, a result especially pertinent to the optical and magnetic properties of the glasses. The fractal dimensionality 4${\mathrm{C}}_{11}$/B of these glasses, obtained from the elastic stiffnesses determined from ultrasonic wave velocities, ranges between 2.3 and 2.8, indicating that their connectivity tends towards having a three-dimensional character.