Observation of the Phase Transition in KNO₃ Single Crystals

Abstract

The process of the phase transition in KNO₃ 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 B_{I III} appears on the cooler end of the crystal and then the boundary B_{III 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.