Mechanisms of structure transformations in thallous nitrate crystals

Geometrical and kinetic factors in polymorphic transformations of single crystals of thallous nitrate have been investigated by optical and X-ray methods. The transformations orthorhombic $\rightleftharpoons$ rhombohedral can proceed by two distinct mechanisms with different kinetics. The slower kinetics are associated with a mechanism which maintains a definite orientation relation and causes formation of plates of the new phase: it is closely similar to martensitic transformation in alloys. Dislocation etch pits can be produced at the interface. Nucleation rates for the other, 'reconstructive', mechanism were predominantly time-dependent in superheating, and temperature-dependent in supercooling. Nucleation appears to occur at defects. A transformation from orthorhombic to cubic can occur in good crystals at 126.5 $^\circ$C and proceeds by a third, continuous mechanism, despite the similarity of the stable rhombohedral structure to the cubic. It appears that the choice of a transformation mechanism can depend on amount of imperfection and can be quite sensitive to small changes in the difference between the structures.