THE ATOMIC-STRUCTURE OF THE REACTION FRONT AS A FUNCTION OF THE KINETIC REGIME OF A SPINEL-FORMING SOLID-STATE REACTION

MgIn2O4 spinel films of thicknesses up to 4 mu m were grown by topotax ial solid state reactions on MgO(001) substrates. The films were chara cterized by X-ray diffraction, scanning electron microscopy, transmiss ion 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 tr ansmission electron microscopy. A network of misfit dislocations was s hown to accommodate the lattice mismatch of +4.5% along the MgO-MgIn2O 4 interface. This network has to move together with the advancing reac tion front. In thin films, with the reaction in the interface-controll ed regime, the Burgers vectors of the network dislocations are pointin g out of the interface plane. This enables the misfit dislocations to easily glide during the reaction. In thick films, with the reaction pr obably being in the diffusion-controlled regime, Burgers vectors are l ying in the interface plane. In this regime the dislocations have to c limb during the reaction. The slow and energetically expensive climb p rocesses do not limit the reaction rate, since now it is in any case c ontrolled by the slow diffusion process. To explain the transition fro m the one reaction regime to the other, a model dislocation reaction i s discussed.