Mössbauer and optical spectroscopic study of temperature and redox effects on iron local environments in a Fe-doped (0.5 mol% Fe2O3) 18Na2O-72SiO2 glass

Local environments of ferric and ferrous irons were systematically studied with Mossbauer (at liquid He temperature) and uv-vis-near ir 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 O partial pressures from 12.3 to 0.27x10-7 atmospheres. The Mossbauer spectroscopic method identified three types of local environments, which were represented by the Fe3+ sextet, the Fe3+ doublet, and the Fe2+ doublet. The Fe3+ sextet ions were assigned to isolated octahedral ions. Under reducing conditions, the octahedral Fe3+ ions were converted into octahedral ferrous ions. The Fe3+ doublet exists both in octahedral and tetrahedral environment, mainly as tetrahedral sites in the reduced samples. The tetrahedral ions were stable against reduction to ferrous ions. The Fe2+ doublet sites existed in octahedral coordination. Combining results from both spectroscopic studies, the 1120- and 2020-nm optical bands were assigned to octahedral ferrous ions with a different degree of distortion rather than different coordinations. Authors assigned the 375-nm band to the transition of octahedral ferric ions that are sensitive to the change of O 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. 55 refs.