Chemical stability of Ni-enriched nanodomains in alkali borate glasses

The stability of Ni nanodomains observed in low alkali borate glasses (10 mol% alkali oxide) has been investigated at higher alkali contents using Ni K-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. EXAFS data indicate that the Ni nanodomains disappear progressively when adding alkali oxide, whatever the nature of the alkali. This medium range reorganization around Ni is associated with a change of the local site from 6- to 5-fold coordination at 27–30 mol% alkali oxide and 4-fold coordination at higher alkali oxide content. Ni–Ni connections are still present for [5]Ni but no second neighbors are observed for [4]Ni in the EXAFS signal. The changes at both the local and medium range around Ni are likely associated to the disappearance of the large, rigid borate groups with the addition of alkali oxide.