Structure of the topotaxial Mg2SnO4/MgO solid reaction front

The atomic structure of the Mg2SnO4/MgO reaction front obtained by topotaxial solid state reaction is investigated. Due to the difference in lattice parameter the front possesses a positive lattice mismatch of 2.5%, which requires the presence of misfit-accommodating defects that must be able to move together with the advancing reaction front. The nature of these defects is studied in detail. Mg2SnO4 films were grown on MgO(001) single-crystal substrates via a gas-solid reaction between tin-oxide vapour and the MgO crystal at 1200 and 1300 degrees C in air. The films were investigated by XRD, SEM, TEM, SAED and HRTEM. Their growth almost exactly follows the topotaxial orientation relationship MgO(001) II Mg2SnO4(001); MgO[100] II Mg2SnO4[100]. Film growth begins with isolated islands, which later coalesce forming a continuous film. The islands of the film always show a characteristic domain structure with each island typically consisting of four domains. The nature of these domains is analyzed and shown to arise from the positive lattice mismatch between the Mg2SnO4 film and the MgO crystal. Furthermore, this lattice mismatch leads to a characteristic dislocation network at the reaction front with line vectors (100) and Burgers vectors of type a(M)/2[011] forming an angle of about 45 degrees with the plane of the reaction front (a(M) - lattice parameter of MgO). This particular structure of the reaction front is discussed mainly with respect to the ability of the misfit dislocations to easily move by glide on {011} planes together with the advancing reaction front, thus facilitating a rapid interface-controlled solid state reaction.