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

Abstract

The L_III, L_II, and L_I 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 L_III and L_II 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.