TEM Investigations of Spinel‐forming Solid State Reactions: Mechanism, film orientation, and interface structure during MgAl₂O₄ formation on MgO (001) and Al₂O₃ (1 1.2) single crystal substrates

Abstract

Abstract The formation of well‐oriented MgAl 2 O 4 spinel films by solid state reactions between (i) a MgO (001) substrate and an Al 2 O 3 vapour and (ii) a sapphire (1 1 .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 MgAl 2 O 4 films grow in a simple cube‐to‐cube orientation: MgAl 2 O 4 (001) ∥ MgO(001); MgAl 2 O 4 [100] ∥ 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°. On sapphire (1 1 .2) substrates the MgAl 2 O 4 films grow almost in the orientation MgAl 2 O 4 (001) ∥ Al 2 O 3 (1 1 .2); MgAl 2 O 4 [010] ∥ Al 2 O 3 [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° 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 spinelforming reaction because of the different crystallographic conditions at the beginning of the reactions.