Electron paramagnetic resonance and optical absorption spectra of Mn(II) ions in silica sol-gel

Electron paramagnetic resonance (EPR) and optical absorption studies have been made for Mn(II) ions in silica sol-gel. The EPR spectra of Mn(II) ions doped silica gels exhibit a sextet centred at g=2.0001 that depends strongly on the concentration of Mn (II) ions. Both the value of A (99±1 G) and the negative shift (-0.0021 +0.0003 ) in the g-value with respect to the free electron value (2.0023) indicate that the Mn (II) ion is in an ionic environment. Remarkable changes have been observed in the EPR spectra for gels heat treated in the temperature range 373-873 K recorded at room and liquid nitrogen temperatures. From the EPR spectra recorded at 123 K the fine structure parameter D has been evaluated. The calculated D values (0.004 cm -1 for as dried and range between 0.015 to 0.02 cm -1 for the heat treated gels) indicate that for many of the Mn (II) sites the distortions are weak. The optical absorption spectra recorded at room temperature for the as dried and heat treated samples exhibit bands characteristic of Mn (II) ions in an octahedral symmetry. EPR and optical absorption studies reveal that the site symmetry around the transition metal ion is predominantly octahedral.