TEM investigations of spinel-forming solid state reactions: Reaction mechanism, film orientation, and interface structure during MgAl2O4 formation on MgO(001) and Al2O3(11.2) single crystal substrates

The formation of well-oriented MgAl2O4 spinel films by solid state reactions between (i) a MgO (001) substrate and an Al2O3 vapour and (ii) a sapphire (<1(1)over bar .2>) substrate and a deposited solid MgO film, respectively, is experimentally investigated. Composition, structure and morphology of the films are characterized by XRD, SEM, TEM/SAED, and EDX. The reaction fronts involved are investigated by cross-sectional atomic resolution transmission electron microscopy (ARM). The direction of the overall diffusion flux and the kind of diffusing species are determined in experiments using inert markers of sub-micron size. There are common features and, however, distinct differences between cases (i) and (ii). On MgO (001) substrates, the MgAl2O4 films grow in a simple cube-to-cube orientation: MgAl2O4(001) parallel to MgO(001); MgAl2O4[100] parallel to MgO[100]. The films consist of small grains about 25 to 50 nm in diameter, the orientation of which is symmetrically distributed around the exact orientation, with maximum deviations of about +/-2 degrees. On sapphire (<1(1)over bar .2>) substrates the MgAl2O4 films grow almost in the orientation MgAl2O4(001) parallel to Al2O3(<1(1)over bar .2>); MgAl2O4[010] parallel to Al2O3[11.0]. These films consist of larger grains about 100 nm in diameter, the orientation of which systematically deviates from the above orientation by unidirectional rotations up to 5 to 6 degrees around the substrate [11.0] axis. The structures of the reaction fronts show corresponding differences, which are discussed in terms of different mechanisms occurring at the initial stage of the spinel-forming reaction because of the different crystallographic conditions at the beginning of the reactions.