Bonding and structure of Nd3+ in BeF2 glass by XANES and EXAFS spectroscopy

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Abstract
The LIII, LII, and LI XANES and EXAFS of Nd3+ have been measured using synchrotron radiation from SPEAR at Stanford Synchrotron Radiation Laboratory to probe the bonding and structure of Nd3+ in a BeF2 glass containing 4 mol % NdF3. Crystalline NdF3 and Nd2O3 were used as reference compounds to model the chemical environment of Nd3+ in the glass. It is found that in the glass there is a substantial increase in the intensity of the Nd LIII and LII white lines compared with those of crystalline NdF3. EXAFS analysis showed that there is a shrinkage of the innermost Nd–F bond distance and a reduction of nearest neighbor fluorine coordination to ∼7 in the glass compared with nine in pure NdF3. The sevenfold coordination is in agreement with recent molecular dynamics calculations. Using Slater’s atomic shielding constants, the observed increase in white line intensities in the glass was attributed to a covalency effect due to back donation of valence electron from the ligand to the partially filled 4f orbitals of the Nd3+ ions. This study illustrates ability of the combined XANES and EXAFS technique in elucidating the chemical bonding and local structure of a given atomic constituent in glassy solids.