MISFIT ACCOMMODATION MECHANISMS AT MOVING REACTION FRONTS DURING TOPOTAXIAL SPINEL-FORMING THIN-FILM SOLID-STATE REACTIONS - A HIGH-RESOLUTION TRANSMISSION ELECTRON-MICROSCOPY STUDY OF 5 SPINELS OF DIFFERENT MISFITS

Thin films of five different spinels MgAl2O4, MgCr2O4, MgIn2O4, TiMg2O 4 and MgFe2O4 were grown by topotaxial solid state reactions on MgO(00 1) substrates. The spinels widely differed in their lattice parameter resulting in a variation in sign and amount of lattice misfit to the s ubstrate. The films were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy/selected-area el ectron diffraction and energy-dispersive X-ray microanalysis before th e spinel-MgO reaction fronts were investigated in detail by cross-sect ional high-resolution electron microscopy. For almost vanishing misfit (TiMg2O4-MgO and MgFe2O4-MgO) the films were exactly cube-to-cube ori ented to the substrate, with the spinel-MgO reaction front completely coherent. During the reaction, a network of cation antiphase boundarie s formed in these films. For non-vanishing misfit, semicoherent reacti on fronts occurred, with different interfacial defects forming, depend ing on sign and amount of the spinel-MgO lattice misfit. For negative misfit (MgAl2O4-MgO and MgCr2O4-MgO), the interfacial dislocations wer e edge dislocations, with their Burgers vectors lying in the interface plane. Together with the advancing reaction front, they moved by clim b, emitting vacancies into the dense-packed lattices of spinel and MgO . For positive misfit (MgIn2O4-MgO), there was a network of interfacia l edge dislocations, with their Burgers vectors pointing out of the in terface. The Burgers vector component perpendicular to the plane of th e reaction front permits these dislocations to glide in order to cope with the advancing reaction front. Obviously, the system here avoids c limb processes so as to prevent the emission of interstitial atoms int o the densely packed lattices. Such a process would be unfavourable un der both energetic and kinetic aspects. Owing to the perpendicular Bur gers vector component the MgIn2O4 films consist of domains that are ti lted by an angle of 3.5 degrees off the exact cube-to-cube orientation around four different [110] axes. The results are discussed in terms of the interplay between the interfacial reaction mechanism and the pr operties of the interfacial dislocations moving together with the adva ncing reaction front.