The atomic structure of the reaction front as a function of the kinetic regime of a spinel-forming solid-state reaction

摘要

MgIn2O4 spinel films of thicknesses up to 4 mu m were grown by topotaxial solid state reactions on MgO(001) substrates. The films were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy-selected-area electron diffraction and energy-dispersive X-ray microanalysis. The atomic structure of the spinel-MgO reaction front was investigated by cross-sectional high-resolution transmission electron microscopy. A network of misfit dislocations was shown to accommodate the lattice mismatch of +4.5% along the MgO-MgIn2O4 interface. This network has to move together with the advancing reaction front. In thin films, with the reaction in the interface-controlled regime, the Burgers vectors of the network dislocations are pointing out of the interface plane. This enables the misfit dislocations to easily glide during the reaction. In thick films, with the reaction probably being in the diffusion-controlled regime, Burgers vectors are lying in the interface plane. In this regime the dislocations have to climb during the reaction. The slow and energetically expensive climb processes do not limit the reaction rate, since now it is in any case controlled by the slow diffusion process. To explain the transition from the one reaction regime to the other, a model dislocation reaction is discussed.