Spectroscopic study of the coordination and valence of Fe and Mn ions in and on the surface of aluminas and silicas

Abstract

A comparative study of the first-row transition metal ions Fe3+ and Mn2+ on the surfaces of aluminas and silicas has been made using the conventional techniques e.s.r., X-ray diffraction, magnetic susceptibility and in addition a relatively new technique phosphorescence spectroscopy.The results indicate that Fe3+ ions dissolve readily iny-alumina up to 1 w % in samples calcined at 200°C and up to at least 10 % in samples calcined at 800°C. In silica not more than 0.1 w % Fe3+ could be dissolved, the remainder crystallizing as or-Fe203 on the surface. E.s.r. and phosphorescence indicate that Fe3+ ions ony-A1203 calcined below 400°C are mostly octahedrally coordinated on the surface, but above this temperature the majority of the Fe3+ ions take up tetrahedral coordination. Mn2+ ions almost exclusively occupy tetrahedral holes at the surface. At the phase transition 8-tcc-A1203 above lOOO"C, Fe3+ ions move into trigonally distorted octahedral sites whereas Mn2 + changes valence to Mn4+ in octahedral sites. Also in silica, Fe3+ ions preferentially take up tetrahedral coordination above 400°C heat treatments and remain in these sites up to 1200°C calcination temperatures .Most of these results can be explained on the basis of the differences between the structures of silicas and aluminas.

Cathodoluminescence Colours of α-Quartz

“…It appears, therefore, that the origin of the yellow luminescence in authigenic e-quartz is related to trace elements incorporated during crystallization from oxidizing solutions. However, the observation by Pott and McNicol (1971) of a red luminescence colour with peaks at 695 and 727 nm in quartz containing ferric iron (0.01 to 0.1% Fe 3+) suggests that ferric iron is not the primary cause of yellow luminescence.…”

“…The phenomenon has been attributed to the presence of trace elements (Claffy and Ginther, 1959;Pott and McNicol, 1971;Mitchell and Denure, 1973;Sprunt, 1981), to lattice ordering (Zinkernagel, 1978), to the occurrence of defect centres (Sippel, 1971;Mitchell and Denure, 1973;Jones and Embree, 1976;Koyama et al, 1977;Gee and Kastner, 1980) or to an intrinsic property of the SiO 4 tetrahedron (Hanusiak and White, 1975;Trukhin and Plaudis, 1979). The trace amounts of elements (other than silica and oxygen) incorporated in a-quartz, as well as the chargecompensating exchange of silica by aluminium or iron with hydrogen, lithium, sodium or potassium, and the additional lack of oxygen, are the main reasons for the difficulty in determining the origin of a-quartz luminescence.…”

“…Moreover, the red luminescence colour was still present even after 1 k hours of continuous electron bombardment. Sprunt (1981) and Pott and McNicol (1971) concluded that ferric iron in trace amounts in the quartz lattice causes a red luminescence colour with a broad double-peak at 645 nm and at 725 nm.…”

The problem of dating quartz 1: Spectroscopic ionoluminescence of dose dependence

“…Deconvolution indicated that this emission is a composite of at least two emissions at 1.7-1.8 eV and 2.0 eV, which are attributed to Fe 3+ impurities and non-bridging oxygen hole centres (NBOHC) respectively. The substitution of Si 4+ with tetrahedral Fe 3+ , has been associated with emissions at 1.75 eV (Kempe et al, 1999;Pott and McNicol, 1971) and 1.65 eV (Stevens-Kalceff, 2009). Red-IR (1.7 eV) luminescence in feldspar, another framework silicate, has also been linked to tetrahedral Fe 3+ using ESR (Finch and Klein, 1999).…”

“…Such a model does not involve changes in the luminescence centre populations, rather that these are now accessible to the excited energy. If we accept the latter model, then luminescence from Fe 3+ centres (Kempe et al, 1999;Pott and McNicol, 1971;Stevens-Kalceff, 2009) may indeed be the cause of the red luminescence, perhaps also associated with NBOHC formation. The contributions to red luminescence from these two types of centre may, in due course, be deconvolved by time-resolved luminescence spectroscopies.…”