X-ray-absorption spectroscopic studies of sodium polyphosphate glasses

Abstract

A series of sodium polyphosphate glasses of composition ${\mathrm{Na}}_{\mathit{n}+2\mathrm{}}$ ${\mathrm{P}}_{\mathit{n}}$ ${\mathrm{O}}_{3\mathit{n}+1\mathrm{}}$ have been studied using synchrotron-radiation x-ray-absorption spectroscopy. High-resolution P L-edge and K-edge x-ray appearance near-edge structure (XANES) as well as K-edge extended x-ray-absorption fine structure (EXAFS) of the phosphates have been investigated. It has been found that while the K-edge XANES spectra of different phosphate glasses are similar, the L-edge XANES features are strikingly different. These results show that the intensities of the low-energy spin-orbit doublet in the P L-edge spectra increase with average chain length of polyphosphate glasses. The increase is very rapid up to n=20, then it levels off as the chain length increases above this value. Using this information, a quantitative calibration curve has been established which can be used to determine the chain length of polyphosphates in bulk or surface films. The features observed in the XANES spectra of the polyphosphates have been assigned. From EXAFS measurements, the average bond distances and coordination numbers around P have been measured. It was found that within the accuracy of EXAFS, average P-O bond distances in polyphosphate glasses do not change substantially.