Symmetry analysis of the atomic mechanism of ferroelastic switching and mechanical twinning. Application to thallous nitrate

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Abstract
The various approaches used for calculating the displacements undergone by atoms during mechanical twinning and ferroelastic switching are reviewed. It is emphasized that a rigorous group-theoretical approach is usually essential for completeness and for dealing with complex situations. The maturity gained over the years, as well as some recent advances, in the symmetry description of domain structures of ferroic and other crystals can be directly exploited for this purpose. Ferroelastic switching in TINO3 is analysed as an illustration of the concepts discussed. This crystal belongs to the Aizu species m3mFmmm(ss) at room temperature, and has a primitive unit cell eight times larger than that of its cubic prototype. A coset decomposition analysis of the prototype space group Pm3m is carried out with respect to the space group Pnma of the ferroelastic phase to determine the 48 different possible domain types. Atomic displacement vectors involved in the switching from one domain to another are worked out to provide a rationalization of the observed easy mode of ferroelastic switching across {221} planes.

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