The influence of aluminium on iron oxides: X. properties of Al-substituted goethites

Abstract

AbstractFifty-seven goethites, synthesized by a variety of procedures and with Al substitutions of 0–33 mole%, were characterized by XRD, IR, DTA, TEM and chemical techniques. Most of the properties measured showed significant intercorrelations. Mole% Al substitution (measured chemically) did not explain all the relationships among variables, but the inclusion of Δa, defined as the observedadimension minus theadimension predicted by the Vegard rule, explained much of the variation not explained by Al substitution. OH stretching frequency, in particular, was better correlated with Δathan the Al substitution or other properties. The properties of the goethites could best be explained by a combination of Al substitution and structural defects, with Δabeing a measure of the defects. In general, the effect of structural defects was opposite to that of Al substitution. Increase in Al substitution led to a decrease in all three unit-cell dimensions and OH stretching frequency and to an increase in the distance between the two OH bending vibrations (δOH-γOH) and the temperature of dehydroxylation. Increase in structural defects, on the other hand, caused theadimension, Δa, and OH stretching frequency to increase andδOH-γOH and the average temperature of dehydroxylation to decrease. Crystal size tended to decrease with increases in both Al substitution and structural defects. Surface area was significantly correlated with the reciprocal of the mean crystal thickness in theadirection. Comparison of XRD and TEM data showed that many samples consisted of crystals with several coherently scattering domains. The nature of the defects, i.e. whether they occur primarily in the interdomain areas or whether they are also distributed throughout the coherently diffracting domains, could not be determined.