Date of Award

9-7-2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Physics & Astrophysics

First Advisor

Gaya Kanisha Marasinghe

Abstract

The atomic-scale structure of four series of binary rare earth phosphate glasses of composition (R2O3)x (P2O5)1-x, where R = Nd, Pr, Eu, and Sm were investigated. Compositions of praseodymium phosphate glasses ranged from polyphosphate to ultraphosphate (0.108 < x < 0.295), while neodymium phosphate compositions varied from metaphosphate to ultraphosphate (0.204 < x < 0.255). Samarium phosphates and the two europium phosphate glasses investigated were ultraphosphates with very low rare earth contents (x < 0.15). The R3+ local environment of these glasses was investigated using extended X-ray absorption fine structure (EXAFS) spectroscopy. A series of neodymium borate glasses with the general formula (Nd2O 3)x(B2O3)1-x (0.01 < x < 0.35) was also analyzed. For both neodymium and praseodymium phosphate glasses the nearest neighbor oxygen coordination decreases with increasing rare earth concentrations. For the first oxygen shell the R-O distance ranged between 2.38-2.40 Å and 2.39-2.46 Å for neodymium and praseodymium glasses, respectively. The second coordination shell consisted of phosphorus atoms, with R-P distance about 3.4-3.5 Å and coordination numbers (CN) ranged from 1.5 to 3. There was an oxygen shell (a second oxygen shell) at R-O distance 4.1 Å for both series. In this composition range, there is no direct evidence of short R-R distances by clustering of rare earth atoms. But first shell R-O CN's obtained suggest sharing of some of the oxygens by two rare-earth ions when the composition x is greater (approximately) than 0.17. Samarium ultraphosphates and europium ultraphosphates (x < 0.15) had first shell R-O distances between 2.38 Å and 2.43 Å. R-P and second R-O shell positions were approximately at 3.7-3.8 Å and at 4.5-4.65 Å respectively. There is no evidence of rare earth clustering in these samples with low rare earth concentrations. First R-O shell CN and the bond distance decreases with the increasing RE concentration for neodymium borate glasses following the same trend as in phosphate glasses. R-B coordination is much higher than the R-P CN's obtained for phosphates.

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