Mossbauer and optical spectroscopic study of temperatureand redox effects on iron local environments in a Fe-doped(0.5 mol% Fe2O3)18Na2Oâ•fi72SiO2 glass

Local environments of ferric and ferrous irons were systematically studied with M€ ossbauer (at liquid helium temperature) and ultraviolet–visible–near infrared spectroscopic methods for various 18Na2O–72SiO2 glasses doped with 0.5 mol% Fe2O3. These were prepared at temperatures of 1300–1600 C in ambient air or at 1500 C under reducing conditions with oxygen partial pressures from 12.3 to 0:27 10 7 atmospheres. The M€ ossbauer spectroscopic method identified three types of local environments, which were represented by the Fe3þ sextet, the Fe3þ doublet, and the Fe2þ doublet. The Fe3þ sextet ions were assigned to isolated octahedral ions. Under reducing conditions, the octahedral Fe3þ ions were readily converted into octahedral ferrous ions. The Fe3þ doublet exists both in octahedral and tetrahedral environment, mainly as tetrahedral sites in the reduced samples. The tetrahedral ions were found stable against reduction to ferrous ions. The Fe2þ doublet sites existed in octahedral coordination. Combining results from both spectroscopic studies, the 1120and 2020-nm optical bands were assigned to octahedral ferrous ions with a different degree of distortion rather than different coordinations. Further, we assigned the 375-nm band to the transition of octahedral ferric ions that are sensitive to the change of oxygen partial pressure in glass melting and 415-, 435-, and 485-nm bands to the transitions of the tetrahedral ferric ions that are insensitive to oxidation states of the melt. The effect of ferric and ferrous ions with different coordination environments on the glass immiscibility was elucidated. 2002 Elsevier Science B.V. All rights reserved.