Orientation relationships between complex low symmetry oxides: Geometric criteria and interface structure for yttrium aluminate eutectics

Solidified eutectics of yttrium-aluminum perovskite (YAP)-yttrium-aluminum monoclinic (YAM) and YAM-yttria were observed by transmission electron microscopy and by optical microscopy. Orientation relationships (ORs), interphase boundary orientations and some boundary structures were determined and compared to those predicted by various geometric criteria. Six ORs were found in YAP-YAM and two were found in YAM-yttria. Near-coincidence site lattices (NCSLs) were calculated for each eutectic. OR predictions based on NCSL energy minimization were compared to NCSLs for the observed ORs. Predictions by symmetry overlap criteria and the edge-to-edge model were also compared to observations. All common ORs had coincident low-index real-space lattice vectors that were consistent with predictions of the edge-to-edge model. Three of these ORs had matching planes cozonal with the coincident real-space directions that were consistent with the edge-to-edge model. Issues with application of geometric criteria to interphase boundaries between complex, low symmetry materials are discussed.