Misfit accommodation mechanisms at moving reaction fronts during topotaxial spinel-forming thin-film solid-state reactions: A high-resolution transmission electron microscopy study of five spinels of different misfits

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Abstract
Thin films of five different spinels MgAl2O4, MgCr2O4, MgIn2O4, TiMg2O4 and MgFe2O4 were grown by topotaxial solid state reactions on MgO(001) substrates. The spinels widely differed in their lattice parameter resulting in a variation in sign and amount of lattice misfit to the substrate. The films were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy/selected-area electron diffraction and energy-dispersive X-ray microanalysis before the spinel-MgO reaction fronts were investigated in detail by cross-sectional high-resolution electron microscopy. For almost vanishing misfit (TiMg2O4-MgO and MgFe2O4-MgO) the films were exactly cube-to-cube oriented to the substrate, with the spinel-MgO reaction front completely coherent. During the reaction, a network of cation antiphase boundaries formed in these films. For non-vanishing misfit, semicoherent reaction fronts occurred, with different interfacial defects forming, depending on sign and amount of the spinel-MgO lattice misfit. For negative misfit (MgAl2O4-MgO and MgCr2O4-MgO), the interfacial dislocations were edge dislocations, with their Burgers vectors lying in the interface plane. Together with the advancing reaction front, they moved by climb, emitting vacancies into the dense-packed lattices of spinel and MgO. For positive misfit (MgIn2O4-MgO), there was a network of interfacial edge dislocations, with their Burgers vectors pointing out of the interface. The Burgers vector component perpendicular to the plane of the reaction front permits these dislocations to glide in order to cope with the advancing reaction front. Obviously, the system here avoids climb processes so as to prevent the emission of interstitial atoms into the densely packed lattices. Such a process would be unfavourable under both energetic and kinetic aspects. Owing to the perpendicular Burgers vector component the MgIn2O4 films consist of domains that are tilted by an angle of 3.5° 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 properties of the interfacial dislocations moving together with the advancing reaction front.