Secondary epitaxial orientations in high lattice mismatch systems: A case study for YBa2Cu3O7-δ and SrO thin films deposited on (0 0 1)MgO

•Heteroepitaxial films in ionically bonded structures can contain secondary orientations.•We find that there is a relationship between fractional mismatch or misfit with grain boundary twist angles.•A range of systems have been examined e.g. YBCO and SrO on [0 0 1] MgO.•A novel equation has been derived using the coincident site lattice theory (CSL)•This equation can predict possible in-plane twist boundaries in high lattice mismatch systems. High lattice mismatch epitaxy is receiving wide attention with demands increasing for bonding together materials of differing types to manage defect content. One interesting aspect of heteroepitaxial growth is in the formation of secondary epitaxial orientations in, often, high lattice mismatch systems. Two examples explored here are the cases of epitaxial thin films of YBa2Cu3O7-δ (YBCO) grown onto (0 0 1)MgO (8.55% lattice mismatch) and SrO on (0 0 1)MgO (12.2% lattice mismatch). Under certain conditions, these materials have been found, using electron microscopy, to contain large twist boundary rotated grains which adopt well defined orientations. In the case of YBCO (SrO) on MgO the most frequent in-plane orientations observed occur at 23.5° (27.4°) on MgO, respectively. These values show that there is an increase in the twist angle of the most frequently observed grain orientations with lattice mismatch as observed experimentally. These have been explained, in the present work, in terms of a novel means of describing the coincident site lattice theory. Indeed, an equation has been derived using this novel theory which closely matched these experimental observations.