Interstitial Transition Metal Impurities as a Possible Cause of Enhanced Reactivities

Abstract

Trivalent ions of Fe, Cr and Mo are easily incorporated into CsCl‐type lattices. Evidently their geometries partly derive from those for divalent ions by exchange of a neutral molecule by hydroxyl or halide. In the ammonium halides hydrogen bonding manifests itself in predominance of low site symmetries and dynamic effects. Evidence for interstitial incorporation of di‐ and trivalent ions in quartz and isomorphous ABO4 compounds is also presented. EPR data for Fe3+ indicate simultaneous incorporation of O2− or OH− in the c‐axis channels. A kinetic reason for preferred interstitial incorporation is proposed. Enhanced mobilities of these as well as certain main constituents may also enhance the reactivities.

The structures of three centers of trivalent iron in ?-quartz

“…This axis of compression along e is a common feature also for Fe 3+ in the AB04 isomorphs of quartz (A=A1, Ga; B-P; As) (Krauss and Lehmann 1975) whereas the monoclinic site symmetries only require the twofold axis to coincide with one of the principal axes. As shown by SPM analyses (Behner et al 1986) neither the substitutional nor the interstitial site on the twofold axis lead to such a ZFS pattern. With occupation by a Fe 3 + ion considerable relaxation of their geometries must be expected for both sites (due to the large size mismatch for the substitutional site), but it is extremely unlikely that it results in such a special orientation of the ZFS tensors.…”

“…The controversy about this so-called I center is not yet settled. Pseudosymmetry arguments (Michoulier and Gaite 1972;Gaite 1980) seem to favor the substitutional model (Scala andHutton 1976, Mombourquette et al 1986) while a number of other observations including the ZFS pattern (Behner et al 1986) favor the interstitial one.…”

EPR of iron centres in silicon dioxide

“…For a detailed discussion of the problem, one can consult various publications I7, 14,15,61,62,66,75].…”