Unambiguously determine the microscopic ferroelastic domain-wall mechanism in CST

Establish unambiguously which microscopic pathway connects neighbouring ferroelastic domains in Ca$_{2.15}$Sr$_{0.85}$Ti$_2$O$_7$, specifically whether the wall involves suppression of the X$_2^+$ rotation accompanied by continuous rotation of the X$_3^-$ tilt or an alternative pathway, using higher-resolution mode-resolved mapping and/or complementary techniques.

Background

The study identifies approximately 120-nm-wide ferroelastic domain walls in Ca$_{2.15}$Sr$_{0.85}$Ti$_2$O$_7$ and finds evidence that the X$_3^-$ octahedral tilt rotates continuously through the wall while the X$_2^+$ rotation is likely suppressed. Several symmetry-allowed pathways remain possible, including pathways involving tetragonal intermediates, and the available spatially averaged mode maps do not resolve the wall-centre structure sufficiently to distinguish them conclusively.

Resolving this problem would determine the microscopic structural mechanism of ferroelastic wall formation and clarify how coupled octahedral order parameters evolve across extended domain walls in hybrid improper ferroelectrics.