Spectroscopic properties of amber chromophore in silicate glasses

Abstract The structure of the amber chromophore, a Fe 3+ oxysulfide complex, is investigated in a suite of soda‐lime glasses, using optical absorption spectroscopy (UV‐Vis‐NIR), Raman spectroscopy, and electron paramagnetic resonance (EPR). These spectroscopic data on glasses containing a similar Fe‐content show an outstanding linear variation as a function of S concentration. This chemical dependence allows us to assign a composite Raman band at 420 cm −1 to the presence of Fe‐S bonds. EPR data indicate that the classical EPR signal of Fe 3+ in glasses coexists with another signal showing an axial distortion of the Fe 3+ site, due to O 2− ‐S 2− substitution. These data support the presence of tetrahedral Fe 3+ in a mixed ligand configuration with three O and one S neighbors. The two optical absorption bands observed in the UV range, near 24,200 and 34,000 cm −1 , are due to S 2− —Fe 3+ and O 2− —Fe 3+ charge transfer transitions within the chromophore, respectively. The former is at the origin of a broad Gaussian‐shaped absorption band, causing a tail that extends into the visible range. The absence of a discrete absorption band in the visible range, together with a continuously increasing absorption with increasing wavenumbers, causes the original brown color of amber glasses. The amber chromophore concerns only a minority of Fe 3+ sites, as shown by EPR. However, as the electronic transitions associated with charge transfer are allowed and intense, the amber chromophore has an efficient coloring power. The Fe 2+ optical absorption spectrum remains identical to that observed in soda‐lime glasses containing iron, without specific site distortion, showing the absence of substituted sulfide ligands in the coordination site of Fe 2+ . The control of the melting atmosphere conditions for chromophore formation plays a crucial role. Glass melting experiments in platinum or graphite crucibles are consistent with the literature, indicating that glasses develop an amber color only under intermediate redox conditions, estimated to correspond to an oxygen partial pressure in the range of 10 −10 to 10 −8 atm. Above and below this range, S 2− and Fe 3+ do not coexist, and the glass is colorless.