Sol-gel preparation and structural investigations of silico-phosphate glasses doped with Fe ions

Different aspects related to the sol-gel preparation and structural investigation of silico-phosphate glasses (SiO₂–P₂O₅ system) doped with Fe ions are reported. During the preparation process, tetraethoxysilane was used as a precursor for SiO₂ and phosphoric acid (H₃PO₄-HP) for P₂O₅. Ferric chloride was used as precursor for Fe ions, water as reagent for hydrolysis reaction and ethylic alcohol as solvent. The pH of the sols was modified by adding hydrochloric acid and ammonia. It was observed that a slight increase of the solution temperature (up to 40 °C) allows a drastic decrease of the gelation time (from days to hours). The structure of the obtained powders dried in air at room temperature and at 100 °C for 10 h and subsequently thermally treated at different temperatures was investigated by Fourier transformed infrared (FTIR) and Raman spectroscopy. Vibration modes specific to Si–O–Et, Si–OH, P–O–P, P–O–Si, hydrogen bonds and H₂O, as well as combined modes have been observed. The local structure and electron configurations of the doping Fe ions have been investigated by ⁵⁷Fe Mössbauer spectroscopy.

Mössbauer spectra of the sample with molar composition 0.85SiO₂ 0.08P₂O₅ 0.07Fe₂O₃ (Fe15) annealed at 300 °C collected at 5.7/6 K, 30 K, 60 K and at room temperature

The magnetic pattern is assigned (by its hyperfine parameters) to α-Fe₂O₃ (36 % from total Fe, the rest being dispersed in the matrix as paramagnetic Fe with octahedral oxygen configuration).