Microscopic structure of stacking faults in Sr2NaNb5O15

Stacking faults and other topological defects in ferroics can have a significant influence on the electronic and mechanical properties of the material. Here, regular stacking faults in the tetragonal tungsten bronze material Sr2NaNb5O15 are investigated through transmission electron microscopy, symmetry mode analysis, and machine-learned force-field calculations. It is shown that the faults, with a fault vector of 14 [212] ¯ o, annihilate in sets of four in the material, owing to the ¼ unit cell displacement along the b-axis. The four resulting domains emerge as four possible directions of the S3 order parameter, related to NbO6 octahedral tilts in the material. Force-field calculations reveal that the stacking faults are likely placed at positions where the octahedra in neighboring domains have similar magnitudes of rotation, and that the estimated stacking fault energy is 46 mJ/m2. The investigation shows that the stacking faults have a local effect on the in-plane polar mode present in the structure, and therefore could affect the ferroelectric properties.