The process of the phase transition in KNO
3 single crystals have been observed through polarizing microscope with a heating stage which can set up a large temperature gradient across the crystal plate. On heating, the transition boundary B
II I (from phase II to phase I) starts from the hot end of the crystal and proceeds to the other end making the specimen somewhat polycrystalline. On further heating, recrystallization starts at about 250°C and the front of the recrystallization sweeps over the crystal making the microscope view a little more single crystal-like. On cooling, the boundary
BI III appears on the cooler end of the crystal and then the boundary
BIII II appears in quick succession. The boundary B
III II usually leaves many small cracks after its passage. However, the speed of the boundary B
I III is rather small as compared with that of the boundary B
III II, and shortly after their appearance the boundary B
III II often comes up with the boundary B
I III. Thereafter the united boundary B
I II proceeds further to the hot end leaving no more cracks.
View full abstract