The Microscopic Structure of Stacking Faults in Sr$_2$NaNb$_5$O$_{15}$

TL;DR

This study investigates the microscopic structure of stacking faults in Sr$_2$NaNb$_5$O$_{15}$ using microscopy, symmetry analysis, and machine learning, revealing their formation, domain structure, and potential impact on ferroelectric properties.

Contribution

It provides a detailed characterization of stacking faults in Sr$_2$NaNb$_5$O$_{15}$, including their formation mechanism, domain configurations, and energetic stability, using combined experimental and computational methods.

Findings

Stacking faults annihilate in sets of four due to unit cell displacement.

Four domain orientations correspond to different S$_3$ order parameter directions.

Stacking fault energy estimated at 46 mJ/m$^2$, affecting local polar modes.

Abstract

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 Sr$_2$NaNb$_5$O$_{15}$ 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 $\frac{1}{4}[\bar{2}12]_o$, annihilate in sets of four in the material, owing to the $\frac{1}{4}$ unit cell displacement along the b-axis. The four resulting domains emerge as four possible directions of the S$_3$ order parameter, related to NbO$_6$ octahedral tilts in the material. Force-field calculations reveal that the stacking faults are likely placed at positions where the octahedra in neighbouring domains have similar magnitudes of rotation, and that the estimated stacking fault energy is 46 mJ/m$^2$. The investigation shows that the stacking faults have a significant local effect on the polar modes present in the structure, and therefore could affect the ferroelectric properties.