Nanocrystalline, porous periclase aggregates as product of brucite dehydration

Transmission electron microscopy (TEM) techniques were employed to in situ study the electron-beam induced dehydration of brucite Mg(OH)(2). Under the electron beam, the hexagonal platelets of brucite immediately decompose an d show a morphological shrinkage of 5% and 10-20% in the a and c directions , respectively. The volume contraction occurs first in the rim and then aff ects the center of grains. Electron energy low-loss spectra reveal a simult aneous change in the local mass thickness of 50-55%. Combining these data, it follows that the porosity in the dehydrated material is 37.5-50%. The de composition product is composed of numerous, tiny MgO crystallites and void s. Electron diffraction reveals a topotactic relationship between brucite a nd MgO with [0001](Bru) // [111](MgO) and [11 (2) over bar0](Bru) // [1 (1) over bar0](MgO). Since the porosity of the dehydrated material is slightly smaller than the maximum theoretical porosity (54%), only a small fraction of the voids is transported out of aggregates. Information on the local environment of the oxygen atoms was derived from e xtended energy-loss fine (EXELFS) and energy-loss near-edge structures (ELN ES). In the time course of dehydration the coordination number of oxygen sh ows the expected increase from 3 for brucite to 6 for MgO. In a transient s tate the Debye-Waller factor reaches a maximum, indicating a highly disorde red intermediate state. These data allow us to model the water loss and to examine reaction kinetics applying the Avrami equation. The decomposition o f brucite is interpreted as a complex three-stage process: (i) It proceeds first via an interface-controlled process, starting at the rim of brucite; water escapes through the basal plane. (ii) The dehydrated lattice collapse s then at the rim, whereas the core region is still hydrated. To further de hydrate the grain, the voids have to interconnect and rearrange in the form of a network slowing down the decomposition. At this stage, the process is diffusion-controlled (iii) Finally, the pores are interconnected and reach the surface. The dehydration accelerates and is again an interface-control led process.