The site occupancy of Mg in the brownmillerite structure and its effect on hydration properties: an X-ray/neutron diffraction and EXAFS study

Abstract

Samples of pure (Ca2FeAlO5) and lightly doped (Ca2Fe0.95Al0.95Mg0.05Si0.05O5) brownmillerite have been synthesized. Synchrotron X-ray and neutron diffraction data have been collected so that the structures can be refined using, simultaneously, both diffraction data sets and known compositional information; this overcomes the problem of under-determinacy resulting from multi-occupation of the tetrahedrally and octahedrally coordinated sites in the structure. For the pure form, a 2.7:1 iron/aluminium preference for octahedral/tetrahedral (respectively) occupation is obtained. This trend is reflected also in the doped brownmillerite, though, because of the low level of Mg doping, the occupancy of Mg is only resolved through the additional use of Mg EXAFS (extended X-ray absorption fine structure) data, which shows that Mg displays a distinct octahedral site preference rather than a disordered occupation between the octahedral/tetrahedral sites. The consequences of Mg doping are then examined using time-resolved multi-angle energy-dispersive powder X-ray diffraction studies of the mineral undergoing hydration; this shows that the pure form is more active than the doped form.