EXAFS and x-ray structural studies of (Tb2O3)0.26(P2O5)0.74 Metaphosphate Glass

Although the metaphosphate glasses R(PO3)3 containing high concentrations of rare-earth R3+ ions have potentially important applications in optical communications and laser technologies, there is little previous information about their structures. To establish the essential features of the structure of such a glass, the complementary techniques of x-ray diffraction and extended x-ray-absorption fine structure (EXAFS) have been used upon the terbium metaphosphate glass, having a composition found by electron probe microanalysis to be (Tb2O3)0.26(P2O5)0.74. The combination of the two techniques allows for a more comprehensive interpretation of the structural data, since each method suffers from differing analytical problems, notably the composite radial distribution function obtained from diffraction, and the present lack of a good calibrant for the EXAFS experiment. The diffraction results prove to be consistent with a network model dominated by a phosphate glass skeleton constructed from PO4 tetrahedra bonded to adjacent tetrahedra via bridging oxygen atoms. This previously unestablished structural model has been used formerly to interpret a wide range of thermal, ultrasonic, and optical properties. The EXAFS, being an atom-type specific probe, have provided the more direct information on the local structure at the rare-earth ion site in vitreous (Tb2O3)0.26(P2O5)0.74: notably a strong correlation in 2.25 Å, associated with an average coordination number of 7±1 for oxygen atoms surrounding a Tb3+ ion