The structure of Mg 2SnO 4/MgO topotaxial reaction fronts during gas–solid and solid–solid reactions

Abstract

Thin spinel films were grown on MgO(001) substrates by a surface reaction between the MgO substrate and (i) a SnO 2 vapour, or (ii) a solid SnO 2 film at temperatures around 1200°C. Regime (i) is called a “gas–solid reaction’’, regime (ii) a “solid–solid reaction’’. Investigations by SEM, XRD, TEM/SAED, and HRTEM revealed the films obtained by the gas–solid reaction to grow in almost [001] orientation and to develop a specific morphology. They are composed of domains, the crystal lattices of which are tilted by less than 1° off the overall orientation around two different 〈110〉 axes. A network of interfacial dislocations with Burgers vectors a/2 [011] and a/2 [101] accommodates the Mg 2SnO 4/MgO lattice misfit of +2.5%. The films obtained by the solid–solid reaction grow in the very [001] orientation and do not consist of domains. Here, a network is proven of interfacial dislocations with Burgers vectors a/2 [110] and a/2 [1 1 0], which are parallel to the reaction front. The observations are discussed in terms of the interplay between reaction kinetics, the properties of the misfit-accommodating interfacial dislocations persisting at the moving reaction front, and the differences between gas–solid and solid–solid reactions concerning the starting conditions of the growing spinel phase.